I'm specifically referring to this question:
DDD - How to implement factories
The selected answer has stated:
"factories should not be tied with dependency injection because domain objects shouldn't have dependencies injected into them."
My question is: what is the reasoning of not being able to inject dependencies in to your entities? Or am I just misunderstanding the statement? Can someone please clarify?
Kind of old, but I really want to address this because I've been running into this a lot, and express my opinion on it:
I often hear that Domain Objects should not "depend" on things. And this is true. I often see people extrapolate this to mean we should not inject things into domain object.
That's the opposite of what dependency means. The domain shouldn't depend on other projects, this is true. But, the domain can define its own interfaces which other projects may then implement, which can then be injected back into the domain. As we all know, that's what is called Dependency Inversion (DI).
That is literally the opposite of having a dependency. Not allowing DI into the domain completely hamstrings your ability to accurately model the domain, forces odd SRP violations, and pretty much kills the usability of domain services.
I really feel like I must be the crazy one here, because I feel like everyone reads "the Domain must not have any dependencies" then thinks "being injected with something means you are dependant on it" and comes to the conclusion "therefore we can't inject dependencies into the domain.
That leaves us with the wonderful logic:
Dependency Inversion == Dependency
Domain Objects aren't Factories, Repos, etc. They are only Entities, Value Objects, Domain Services and Aggregate Roots. That is, they must be classes which encapsulates the data your business domain uses, the relationships between them, and the behaviour(read modifications) that the domain can do on that data.
Repository is a pattern to abstract away the persistence infrastructure you are using. It's in DDD because it makes your app decoupled from your database, but not all DDD app need or even should use repository.
Factory is a pattern to isolate the construction logic of objects. It's also just a good practice that DDD recommends, but not really needed in all scenarios.
Domain Objects shouldn't depend on anything else, because they are the core of your app. Everything will depend on them. So keeping them free of other dependency makes a clear one way dependency chain, and reduces the dependency graph. They are the invariants, the model, the foundation. Change them, and you probably need to change a lot of stuff. So changing other things shouldn't force them to change.
Domain objects should not have many dependencies.
By Fowler's Tell-Don't-Ask principle (https://martinfowler.com/bliki/TellDontAsk.html), you would want the domain objects to do as much as possible. Including having dependencies. But in Clean Code (Uncle Bob) Chapter 6, it mentions it can be a good design to have data structures operated on by procedure/function classes (services). As long as you don't have hybrid objects which combine simple getters/setters as well as more complex tell-don't-ask operations.
Fowler disagreed with thin models and called it an antipattern - AnemicDomainModel. https://www.martinfowler.com/bliki/AnemicDomainModel.html
I disagree with Fowler. I strongly agree with the following quote from another article about this Fat-Models problem: "Following this logic basically every behaviour would end up in the model classes. This is something that we know (by experience) is a bad idea. Hundreds or thousands of lines of code crammed into a single class is a recipe for disaster. Service Objects grew out of this frustration." - https://tmichel.github.io/2015/09/14/oo-controversies-tell-dont-ask-vs-the-web/
We actually have a project with fat domain models which has this exact problem. As requirements change over time and code gets more complex, a huge, fat model is quite inflexible to perform different operations and handle new requirements. Instead of adding new service workflow paths (classes) acting differently on the same simple data model, you have to make expensive, difficult refactors on the enormous, complicated domain model. It encapsulates the data and prevents anyone from modifying the data in unexpected ways but at the same time, it makes it really difficult for new workflow to manipulate the data in new ways.
What's the point of using Fit/FitNesse instead of xUnit-style integration tests? It has really strange and very unclear syntax in my opinion.
Is it really only to make product owners write tests? They won't! It's too complicated for them. So why should anyone Fit/FitNesse?
Update So it's totally suitable for business-rules tests only?
The whole point is to work with non-programmers, often even completely non-technical people like prospect users of a business application, on what application should do and then put it into tests. While making tests work is certainly too complicated for them, they should be able to discuss tables of sample data filled out in e.g. Word. And the great thing is, unlike traditional specification, those documents live with your application because automated tests force you to update them.
See Introduction To Fit and Fit Workflow by James Shore and follow links to the rest of documentation if you want.
Update: Depends on what you mean by business rules? ;-) Some people would understand it very narrowly (like in business rules engines etc), others---very broadly.
As I see it, Fit is a tool that allows you to write down business (as in domain) use cases with rich realistic examples in a document, which the end users or domain experts (in some domains) can understand, verify and discuss. At the same time these examples are in machine readable form so they can be used to drive automated testing, You neither write the document entirely by yourself, nor requre them to do it. Instead it's a product of callaboration and discussion that reflects growing understanding of what application is going to do, on both sides. Examples get richer as you progress and more corner cases are resolved.
What application will do, not how, is important. It's a form of functional spec. As such it's rather broad and not really organized by modules but rather usage scenarios.
The tests that come out of examples will test external behavior of application in aspects important from business point of view. Yes, you might call it business rules. But lets look at Diego Jancic's example of credit scoring, just with a little twist. What if part of fit document is 1) listing attributes and their scores and then 2) providing client data and checking results, Then which are the actual business rules: scoring table (attributes and their scores) or application logic computing the score for each client (based on scoring table)? And which are tested?
Fit/FitNesse tests seem more suitable for acceptance testing. Other tests (when you don't care about cooperation with clients, users, domain experts, etc., you just want to automate testing) probably will be easier to write and maintain in more traditional ways. xUnit is nice for unit testing and api tests. Each web framework should have some tool for web app/service testing integrated in its modify-build-test-deploy cycle, eg. django has its little test client. You've lots to chose from.
And you always can write your own tool (or preferably tweak some existing) to better fit (pun intended) some testing in your particular domain of interest.
One more general thought. It's often (not always!!!) better to encode your tests, "business rules" and just about anything, in some form of well defined data that is interpreted by some simple, generic piece of code. Then it's easy to use the data in some other way: generate documentation, migrate to new testing framework, port application to new environment/programming language, use to check conformance with some external rules or other system (just use your imagination). It's much harder to pull such information out from code, eg. simple hardcoded unit tests or business rules.
Fit stores test cases as data. In very specific format because of how it's intended to be used, but still. Your domain specific tests may use different formats like simple CSV, JSON or YAML.
The idea is that you (the programmer) defines an easy to understand format, such as an excel sheet. Then, the product owner enters information that is hard to understand for people that is not in the business... and you just validate that your code works as the PO expects running Fit.
The way used in xUnit, can be used for programmers as an input for easy to understand or simple information.
If you're going to need to enter a lot of weird examples with multiple fields in your xUnit test, it will became hard to read.
Imagine a case where you have to decide whether to give a loan to a customer, based on the Age, Married/Single, Amount of Childrens, Wage, Activity, ...
As a programmer, you cannot write that information; and a risk manager cannot write a xUnit test.
Helps reduce redundancy in regression and bug testing. Build manageable repository of test cases. Its like build once and use for ever.
It is very useful during cooperation of the QA and devs teams: QA could show to developer the result of the failed test and a developer will easyly help to solve an environment issue and will understand steps for reproducing a bug.
It is suitable for UI and even for API testing.
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I have read a lot of articles on Dependency Injection as well as watched a lot of videos, but I still can't get my head around it. Does anyone have a good analogy to explain it?
I watched the first part of the Autumn of Agile screencast and still was a little confused.
Analogy? I'll give it a whack... Your CD Player stereo is useless without a CD with music on it... (It's dependent on the CD). If they built CD Players with the CD already in it, it would get boring very quickly...
So they build them so you can "inject" the CD, (on which it is dependent) into the player. That way you can inject a different one each time, and get "different" behavior (music) dependent on which one you inject.
The only requirement is that the CD must be compatible with the interface defined by the player. (You can't play a blue-ray disk in a 1992 CD player.)
The best analogy I can think of is that of hiring a mechanic.
Without dependency injection, you hire a mechanic and the mechanic brings his own tools. He may have lousy tools, he may have great tools, he may be using a pipe wrench when he should be using a socket. You don't know, and may not care, so long as he gets the work done.
With dependency injection, you hire a mechanic and you provide him with the tools that you want him to do his work with. You get to choose what you consider to be the best or most appropriate tools for the work you are hiring him to do.
Think of it as a realisation of the "Inversion of Control" pattern. I guess, your problem is, you are so used to it, you don't realize it's that simple.
Let's start at the beginning.
In the early days programs followed a given path through the code. The order of the called functions was given by the programmer.
In interactive programs, e.g. mostly ANY program, you can not say, which function is called at what time. Just look at a GUI or website. You can not say, at what time what button or link is clicked. So the "control" of what's happening is no longer at the program, it's at an outer source. The "control" has been inverted. The function is no longer "acting" it is instead "listening". Think of the hollywood principle: "Don't call us, we call you". A listener is a good example for a realisation of this pattern.
IoC is realized by functions or "methods" in the "object oriented world" of today.
"Dependency Injection" now means the same, but not for "methods", which do something, but for "objects", which hold data.
The data is no longer part of the object holding it. It is "injected" into the object at runtime. To stay in hollywood, think of a film star, playing golf to talk about the business, but to keep in shape, she hungers herself down, minimizing her muscle weight and therefore she is only able to carry one club at a time.
So, on the golf course her game would heavily depend on the one club, she is carrying.
Lucky for her, there are caddies, carrying a whole lot of clubs at one time, and also having the knowledge what club to use at what time. Now she is independent of her limited possibility to carry golf clubs. "Don't think about a concrete club to wear, we know them all and give you the right one at the right time".
The film star is the object and the golf clubs are the members of the object. That's dependency injection.
Maybe focus on the "injection" part? When I see that term, I think of syringes. The process of pushing the dependencies of a component to the component can be thought of as injecting into the component.
Just like with the body, when there is something that it needs in the way of medicine (a component that it needs) you can inject it into the body.
In their 2003 JavaPolis presentation (slides), Jon Tirsén & Aslak Hellesøy had an amusing analogy with a Girl object that needs a Boy to kiss. I seem to remember that the BoyFactory is sometimes known as a 'nightclub', but that's not in the slides.
Another analogy: let us say you are a developer and whenever you like you order computer science books from the market directly - you know the sellers and their prices. In fact your company might have a preferred seller and you contact them directly. All this works fine but may be a new seller is now offering better prices and your company wants to change the 'preferred' seller.
At this point you have to make the following changes - update the contact details (and other stuff) so as to use the new seller. You still place the order directly.
Now consider we introduce a new step in between, there is a 'library' officer in the company and you have to go through him to get the books. While there is a new dependency, you are now immune to any changes to the seller: either the seller changes mode of payment or the seller himself is changed, you now simply put an order to the librarian and he gets the books for you.
From Head First Design Patterns:
Remember, code should be closed (to change) like the lotus flower in the evening, yet open (to extension) like the lotus flower in the morning
A DI-enabled object can be configured by injecting behaviors defined in other classes. The original object structure doesn't have change in order to create many variations. The injection can be made explicit by having a class request other worker-classes in its constructor, or it can be less obvious when using monkeypatching in dynamic languages like Python.
Using an analogy of a Person class, you can take a basic human framework, pass it a set of organs, and watch it evolve. The Person doesn't directly know how the organs work, but their behaviors confirm to an expected interface and influence the owner's physical and mental manifestation.
A magician's sleight of hand! What you may think you see may be secretly manipulated or replaced.
Life is full of dependency injection analogies:
printer - cartridge
digital device - battery
letter - stamp
musician - instrument
bus - driver
sickness - pill
The essence of Inversion of Control (of which Dependency Injection is an implementation) is the separation of the use of an object from the management thereof.
The analogy/example I use is an engine. An engine requires fuel to run, i.e. it is depdendent on fuel. However, the engine cannot be responsible for the fuel it needs. It just 'asks' for fuel, and it is provided (typically by a fuel pump in a car).
The analogy starts breaking down when you look too deep, in that an engine doesn't ask for fuel, it is given it by some kind of management element, like an ECU. One might be able to compare the ECU to a container but I'm not certain how valid this is.
Your project manager asks you to write an app.
You could just write some code based on your career experience so far, but it's unlikely to be what your PM wants.
Better would be if your PM dependency injected you with say a spec for the app. Now your code is going to be related to the spec he gives you.
Better if you were told where the source repository was.
Better if you were told what the tech platform was.
Better if you were told when this needed to be done by.
Etc..
I think a great analogy is a six-year-old with a lego set.
You want your objects to be like the lego bricks. Each one is independent of all the others, and yet offers a clear interface for connecting them to the others. When connecting them together, it doesn't really matter exactly which two bricks you hook together so long as they have a matching interface.
Your dependency injection framework is like the six-year-old. He follows the instructions (i.e., your config file, annotations, etc.) to connect specific bricks together in certain ways to make a particular model.
Of course, since the bricks' interfaces are pretty generalized, they can go together in lots of different ways, so it's easy to come up with new sets of instructions which the six-year-old can use to make a completely different model out of the same bricks.
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As I learn more and more about OOP, and start to implement various design patterns, I keep coming back to cases where people are hating on Active Record.
Often, people say that it doesn't scale well (citing Twitter as their prime example) -- but nobody actually explains why it doesn't scale well; and / or how to achieve the pros of AR without the cons (via a similar but different pattern?)
Hopefully this won't turn into a holy war about design patterns -- all I want to know is ****specifically**** what's wrong with Active Record.
If it doesn't scale well, why not?
What other problems does it have?
There's ActiveRecord the Design Pattern and ActiveRecord the Rails ORM Library, and there's also a ton of knock-offs for .NET, and other languages.
These are all different things. They mostly follow that design pattern, but extend and modify it in many different ways, so before anyone says "ActiveRecord Sucks" it needs to be qualified by saying "which ActiveRecord, there's heaps?"
I'm only familiar with Rails' ActiveRecord, I'll try address all the complaints which have been raised in context of using it.
#BlaM
The problem that I see with Active Records is, that it's always just about one table
Code:
class Person
belongs_to :company
end
people = Person.find(:all, :include => :company )
This generates SQL with LEFT JOIN companies on companies.id = person.company_id, and automatically generates associated Company objects so you can do people.first.company and it doesn't need to hit the database because the data is already present.
#pix0r
The inherent problem with Active Record is that database queries are automatically generated and executed to populate objects and modify database records
Code:
person = Person.find_by_sql("giant complicated sql query")
This is discouraged as it's ugly, but for the cases where you just plain and simply need to write raw SQL, it's easily done.
#Tim Sullivan
...and you select several instances of the model, you're basically doing a "select * from ..."
Code:
people = Person.find(:all, :select=>'name, id')
This will only select the name and ID columns from the database, all the other 'attributes' in the mapped objects will just be nil, unless you manually reload that object, and so on.
I have always found that ActiveRecord is good for quick CRUD-based applications where the Model is relatively flat (as in, not a lot of class hierarchies). However, for applications with complex OO hierarchies, a DataMapper is probably a better solution. While ActiveRecord assumes a 1:1 ratio between your tables and your data objects, that kind of relationship gets unwieldy with more complex domains. In his book on patterns, Martin Fowler points out that ActiveRecord tends to break down under conditions where your Model is fairly complex, and suggests a DataMapper as the alternative.
I have found this to be true in practice. In cases, where you have a lot inheritance in your domain, it is harder to map inheritance to your RDBMS than it is to map associations or composition.
The way I do it is to have "domain" objects that are accessed by your controllers via these DataMapper (or "service layer") classes. These do not directly mirror the database, but act as your OO representation for some real-world object. Say you have a User class in your domain, and need to have references to, or collections of other objects, already loaded when you retrieve that User object. The data may be coming from many different tables, and an ActiveRecord pattern can make it really hard.
Instead of loading the User object directly and accessing data using an ActiveRecord style API, your controller code retrieves a User object by calling the API of the UserMapper.getUser() method, for instance. It is that mapper that is responsible for loading any associated objects from their respective tables and returning the completed User "domain" object to the caller.
Essentially, you are just adding another layer of abstraction to make the code more managable. Whether your DataMapper classes contain raw custom SQL, or calls to a data abstraction layer API, or even access an ActiveRecord pattern themselves, doesn't really matter to the controller code that is receiving a nice, populated User object.
Anyway, that's how I do it.
I think there is a likely a very different set of reasons between why people are "hating" on ActiveRecord and what is "wrong" with it.
On the hating issue, there is a lot of venom towards anything Rails related. As far as what is wrong with it, it is likely that it is like all technology and there are situations where it is a good choice and situations where there are better choices. The situation where you don't get to take advantage of most of the features of Rails ActiveRecord, in my experience, is where the database is badly structured. If you are accessing data without primary keys, with things that violate first normal form, where there are lots of stored procedures required to access the data, you are better off using something that is more of just a SQL wrapper. If your database is relatively well structured, ActiveRecord lets you take advantage of that.
To add to the theme of replying to commenters who say things are hard in ActiveRecord with a code snippet rejoinder
#Sam McAfee Say you have a User class in your domain, and need to have references to, or collections of other objects, already loaded when you retrieve that User object. The data may be coming from many different tables, and an ActiveRecord pattern can make it really hard.
user = User.find(id, :include => ["posts", "comments"])
first_post = user.posts.first
first_comment = user.comments.first
By using the include option, ActiveRecord lets you override the default lazy-loading behavior.
My long and late answer, not even complete, but a good explanation WHY I hate this pattern, opinions and even some emotions:
1) short version: Active Record creates a "thin layer" of "strong binding" between the database and the application code. Which solves no logical, no whatever-problems, no problems at all. IMHO it does not provide ANY VALUE, except some syntactic sugar for the programmer (which may then use an "object syntax" to access some data, that exists in a relational database). The effort to create some comfort for the programmers should (IMHO...) better be invested in low level database access tools, e.g. some variations of simple, easy, plain hash_map get_record( string id_value, string table_name, string id_column_name="id" ) and similar methods (of course, the concepts and elegance greatly varies with the language used).
2) long version: In any database-driven projects where I had the "conceptual control" of things, I avoided AR, and it was good. I usually build a layered architecture (you sooner or later do divide your software in layers, at least in medium- to large-sized projects):
A1) the database itself, tables, relations, even some logic if the DBMS allows it (MySQL is also grown-up now)
A2) very often, there is more than a data store: file system (blobs in database are not always a good decision...), legacy systems (imagine yourself "how" they will be accessed, many varieties possible.. but thats not the point...)
B) database access layer (at this level, tool methods, helpers to easily access the data in the database are very welcome, but AR does not provide any value here, except some syntactic sugar)
C) application objects layer: "application objects" sometimes are simple rows of a table in the database, but most times they are compound objects anyway, and have some higher logic attached, so investing time in AR objects at this level is just plainly useless, a waste of precious coders time, because the "real value", the "higher logic" of those objects needs to be implemented on top of the AR objects, anyway - with and without AR! And, for example, why would you want to have an abstraction of "Log entry objects"? App logic code writes them, but should that have the ability to update or delete them? sounds silly, and App::Log("I am a log message") is some magnitudes easier to use than le=new LogEntry(); le.time=now(); le.text="I am a log message"; le.Insert();. And for example: using a "Log entry object" in the log view in your application will work for 100, 1000 or even 10000 log lines, but sooner or later you will have to optimize - and I bet in most cases, you will just use that small beautiful SQL SELECT statement in your app logic (which totally breaks the AR idea..), instead of wrapping that small statement in rigid fixed AR idea frames with lots of code wrapping and hiding it. The time you wasted with writing and/or building AR code could have been invested in a much more clever interface for reading lists of log-entries (many, many ways, the sky is the limit). Coders should dare to invent new abstractions to realize their application logic that fit the intended application, and not stupidly re-implement silly patterns, that sound good on first sight!
D) the application logic - implements the logic of interacting objects and creating, deleting and listing(!) of application logic objects (NO, those tasks should rarely be anchored in the application logic objects itself: does the sheet of paper on your desk tell you the names and locations of all other sheets in your office? forget "static" methods for listing objects, thats silly, a bad compromise created to make the human way of thinking fit into [some-not-all-AR-framework-like-]AR thinking)
E) the user interface - well, what I will write in the following lines is very, very, very subjective, but in my experience, projects that built on AR often neglected the UI part of an application - time was wasted on creation obscure abstractions. In the end such applications wasted a lot of coders time and feel like applications from coders for coders, tech-inclined inside and outside. The coders feel good (hard work finally done, everything finished and correct, according to the concept on paper...), and the customers "just have to learn that it needs to be like that", because thats "professional".. ok, sorry, I digress ;-)
Well, admittedly, this all is subjective, but its my experience (Ruby on Rails excluded, it may be different, and I have zero practical experience with that approach).
In paid projects, I often heard the demand to start with creating some "active record" objects as a building block for the higher level application logic. In my experience, this conspicuously often was some kind of excuse for that the customer (a software dev company in most cases) did not have a good concept, a big view, an overview of what the product should finally be. Those customers think in rigid frames ("in the project ten years ago it worked well.."), they may flesh out entities, they may define entities relations, they may break down data relations and define basic application logic, but then they stop and hand it over to you, and think thats all you need... they often lack a complete concept of application logic, user interface, usability and so on and so on... they lack the big view and they lack love for the details, and they want you to follow that AR way of things, because.. well, why, it worked in that project years ago, it keeps people busy and silent? I don't know. But the "details" separate the men from the boys, or .. how was the original advertisement slogan ? ;-)
After many years (ten years of active development experience), whenever a customer mentions an "active record pattern", my alarm bell rings. I learned to try to get them back to that essential conceptional phase, let them think twice, try them to show their conceptional weaknesses or just avoid them at all if they are undiscerning (in the end, you know, a customer that does not yet know what it wants, maybe even thinks it knows but doesn't, or tries to externalize concept work to ME for free, costs me many precious hours, days, weeks and months of my time, live is too short ... ).
So, finally: THIS ALL is why I hate that silly "active record pattern", and I do and will avoid it whenever possible.
EDIT: I would even call this a No-Pattern. It does not solve any problem (patterns are not meant to create syntactic sugar). It creates many problems: the root of all its problems (mentioned in many answers here..) is, that it just hides the good old well-developed and powerful SQL behind an interface that is by the patterns definition extremely limited.
This pattern replaces flexibility with syntactic sugar!
Think about it, which problem does AR solve for you?
Some messages are getting me confused.
Some answers are going to "ORM" vs "SQL" or something like that.
The fact is that AR is just a simplification programming pattern where you take advantage of your domain objects to write there database access code.
These objects usually have business attributes (properties of the bean) and some behaviour (methods that usually work on these properties).
The AR just says "add some methods to these domain objects" to database related tasks.
And I have to say, from my opinion and experience, that I do not like the pattern.
At first sight it can sound pretty good. Some modern Java tools like Spring Roo uses this pattern.
For me, the real problem is just with OOP concern. AR pattern forces you in some way to add a dependency from your object to infraestructure objects. These infraestructure objects let the domain object to query the database through the methods suggested by AR.
I have always said that two layers are key to the success of a project. The service layer (where the bussiness logic resides or can be exported through some kind of remoting technology, as Web Services, for example) and the domain layer. In my opinion, if we add some dependencies (not really needed) to the domain layer objects for resolving the AR pattern, our domain objects will be harder to share with other layers or (rare) external applications.
Spring Roo implementation of AR is interesting, because it does not rely on the object itself, but in some AspectJ files. But if later you do not want to work with Roo and have to refactor the project, the AR methods will be implemented directly in your domain objects.
Another point of view. Imagine we do not use a Relational Database to store our objects. Imagine the application stores our domain objects in a NoSQL Database or just in XML files, for example. Would we implement the methods that do these tasks in our domain objects? I do not think so (for example, in the case of XM, we would add XML related dependencies to our domain objects...Truly sad I think). Why then do we have to implement the relational DB methods in the domain objects, as the Ar pattern says?
To sum up, the AR pattern can sound simpler and good for small and simple applications. But, when we have complex and large apps, I think the classical layered architecture is a better approach.
The question is about the Active
Record design pattern. Not an orm
Tool.
The original question is tagged with rails and refers to Twitter which is built in Ruby on Rails. The ActiveRecord framework within Rails is an implementation of Fowler's Active Record design pattern.
The main thing that I've seen with regards to complaints about Active Record is that when you create a model around a table, and you select several instances of the model, you're basically doing a "select * from ...". This is fine for editing a record or displaying a record, but if you want to, say, display a list of the cities for all the contacts in your database, you could do "select City from ..." and only get the cities. Doing this with Active Record would require that you're selecting all the columns, but only using City.
Of course, varying implementations will handle this differently. Nevertheless, it's one issue.
Now, you can get around this by creating a new model for the specific thing you're trying to do, but some people would argue that it's more effort than the benefit.
Me, I dig Active Record. :-)
HTH
Although all the other comments regarding SQL optimization are certainly valid, my main complaint with the active record pattern is that it usually leads to impedance mismatch. I like keeping my domain clean and properly encapsulated, which the active record pattern usually destroys all hope of doing.
I love the way SubSonic does the one column only thing.
Either
DataBaseTable.GetList(DataBaseTable.Columns.ColumnYouWant)
, or:
Query q = DataBaseTable.CreateQuery()
.WHERE(DataBaseTable.Columns.ColumnToFilterOn,value);
q.SelectList = DataBaseTable.Columns.ColumnYouWant;
q.Load();
But Linq is still king when it comes to lazy loading.
#BlaM:
Sometimes I justed implemented an active record for a result of a join. Doesn't always have to be the relation Table <--> Active Record. Why not "Result of a Join statement" <--> Active Record ?
I'm going to talk about Active Record as a design pattern, I haven't seen ROR.
Some developers hate Active Record, because they read smart books about writing clean and neat code, and these books states that active record violates single resposobility principle, violates DDD rule that domain object should be persistant ignorant, and many other rules from these kind of books.
The second thing domain objects in Active Record tend to be 1-to-1 with database, that may be considered a limitation in some kind of systems (n-tier mostly).
Thats just abstract things, i haven't seen ruby on rails actual implementation of this pattern.
The problem that I see with Active Records is, that it's always just about one table. That's okay, as long as you really work with just that one table, but when you work with data in most cases you'll have some kind of join somewhere.
Yes, join usually is worse than no join at all when it comes to performance, but join usually is better than "fake" join by first reading the whole table A and then using the gained information to read and filter table B.
The problem with ActiveRecord is that the queries it automatically generates for you can cause performance problems.
You end up doing some unintuitive tricks to optimize the queries that leave you wondering if it would have been more time effective to write the query by hand in the first place.
Try doing a many to many polymorphic relationship. Not so easy. Especially when you aren't using STI.