I am having trouble understanding ORM in Ruby on Rails. From what I understand there is a 1:1 relationship between tables/columns and objects/attributes. So every record is an object.
Also what exactly is a Model? I know it maps to a table.
What I'm really after is a deeper understanding of the above. Thank you in advance for your help
I'm a Web developer going from PHP to Ruby on Rails.
"From what I understand there is a 1:1 relationship between tables/columns and objects/attributes. So every record is an object."
That is not exactly correct, unless you use the term "object" very loosely. Tables are modelled by classes, while table records are modeled by instances of those classes.
Let's say you have a clients table, with columns id (autonum) and name (varchar). Let's say that it has only one record, id=1 and a name="Ford". Then:
The DB table clients will map to the model class Client.
The record will map to a model instance, meaning that you have to create the object and assign it to a variable in order to work with the record. The most common way would be to do ford = Client.find(1)
The two columns of the table will map to methods on the ford variable. You can do ford.id and you will get 1. You can do ford.name and you will get the string "Ford". You can also change the name of the client by doing ford.name = "Chevrolet", and then commit the changes on the database by doing ford.save.
"Also what exactly is a Model? I know it maps to a table"
Models are just classes with lots of very useful methods for manipulating your database. Here are some examples:
Validations: Besides the typical db-driven validations ("this field can't be null") you can implement much complex validations in ruby ("this field must be a valid email" is the most typical one). Validations are run just before you invoke "save" on a model instance.
Relationships: The foreign keys can also be mapped onto models. For example, if you had a brands table (with its corresponding Brand model) associated via a foreign key to your ford client, you could do ford.brands and you would get an array of objects representing all the records on the brands table that have a client_id = 1.
Queries: Models allow you to create queries in ruby, and translate them to SQL themselves. Most people like this feature.
These are just some examples. Active record provides much more functionalities such as translations, scoping in queries, or support for single table inheritance.
Last but not least, you can add your own methods to these classes.
Models are a great way of not writing "spaguetti code", since you are kind of forced to separate your code by functionality.
Models handle database interaction, and business logic
Views handle html rendering and user interaction
Controllers connect Models with Views
ORM in Rails is an implementation of the Active Record pattern from Martin Fowler's Patterns of Enterprise Application Architecture book. Accordingly, the Rails ORM framework is named ActiveRecord.
The basic idea is that a database table is wrapped into a class and an instance of an object corresponds to a single row in that table. So creating a new instance adds a row to the table, updating the object updates the row etc. The wrapper class implements properties for each column in the table. In Rails' ActiveRecord, these properties are made available automatically using Ruby metaprogramming based on the database schema. You can override these properties if required if you need to introduce additional logic. You can also add so-called virtual attributes, which have no corresponding column in the underlying database table.
Rails is a Model-View-Controller (MVC) framework, so a Rails model is the M in MVC. As well as being the ActiveRecord wrapper class described above it contains business logic, including validation logic implemented by ActiveRecord's Validation module.
Further Reading
Rails Database Migrations guide
Rails Active Record Validations and Callbacks guide
Active Record Associations guide
Active Record Query Interface guide
Active Record API documentation
Models: Domain objects such like User, Account or Status. Models are not necessarily supported by a database backend, as for example Status can be just a simple statically-typed enumeration.
ActiveRecord:
Provides dynamic methods for quering database tables. A database table is defined as a class which inherits ActiveRecord class (pseudo-PHP example):
class User extends ActiveRecord {}
//find a record by name, and returns an instance of `User`
$record = User::find_by_name("Imran");
echo $record->name; //prints "Imran"
//there are a lot more dynamic methods for quering
New records are created by creating new instances of ActiveRecord-inherited classes:
class Account extends ActiveRecord {}
$account = new Account();
$account->name = "Bank Account";
$account->save();
There are two pieces here: the ORM and Rails's MVC pattern. ORM is short for "object-relational mapping", and it does pretty much what it says: it maps tables in your database to objects you can work with.
MVC is short for "model-view-controller", the pattern that describes how Rails turns your domain behavior and object representations into useful pages. The MVC pattern breaks down into three chunks:
Models contain a definition of what an object in your domain represents, and how it is related to other models. It also describes how fields and relationships represented in the object map to backing stores (such as a database). Note that, per se, there's nothing about a model which prescribes that you have to use a particular ORM (or even an ORM at all).
Controllers specify how models should interact with each other to produce useful results in response to a user request.
Views take the results created by controllers and render them in the desired way. (By the time you get to your view, you should mostly know what's being rendered, and there should be very little behavior happening.)
The definition from Wikipedia:
Object-relational mapping (ORM, O/RM,
and O/R mapping) in computer software
is a programming technique for
converting data between incompatible
type systems in relational databases
and object-oriented programming
languages. This creates, in effect, a
"virtual object database" that can be
used from within the programming
language.
From a PHP view it will be in the following way(via example)
Connect to the database and get some row from posts table.
Turn that row to an object with attributes like those in the table columns.
If the posts has comments in comments table, you can also do post.comments and you get the comments also as an array of objects as well.
You can define relationships between tables like saying: Posts has_many Comments, a Comment belongs to a post and so.
So basically you are not working with database rows, instead you turn those rows and their relationships to objects with composition or inheritance relationships.
In layman's terms.
A Rails Model is proxy to a table in the database. These models happens to be Ruby classes.
The objects of these classes are proxies to rows in the table of which this model is a proxy.
Finally the attributes of these objects are proxies to the column data for that particular row.
Above is actually the Rails ActiveRecord ORM.
1:1 is not quite correct, since there is object-relation impedance mismatch.
Related
I have a Ruby on Rails app, where I have a Model called “Fruits” which contains an underlying database,
I this DB I have columns like “Type”, “Value” and few other columns, under Type column I have data like “Apple”, “Orange”, and “Mango”.
I have other models like “Apple”, “Orange”, “Mango” all of which do not have an underlying DB but inherits from the model “Fruits” (the one which I mentioned earlier).
In my code when I write Apple.find_by_Value(some value) , how does this returns data related only to Apple ?
You are looking at a Single Table Inheritance (STI). It allows you to use several models for the same table, using the type column to store the name of the model for each record.
Related documentation : http://api.rubyonrails.org/classes/ActiveRecord/Inheritance.html
I want to know how you can solve this with ruby on rails:
there is a core module which provides a class BasePlugin.
Optional plugins inherits (single table inheritance) from this base class.
Example: The FooPlugin from fooplugin module is a external, optional module (provided by a third party).
Since STI is used the migrations for FooPlugin need to live in the fooplugin module.
Result: BasePlugin does not know its whole table, since optional external modules add extra columns.
I am new to ruby on rails, but have developed database based applications in different languages for years.
Question:
Is the above usage of STI possible with ruby on rails?
The STI table contains the intersection of all attributes for both the parent model and all children models. Note that in STI, BasePlugin is a model, not a module. External plugins are generally provided as gems.
But the key thing is that BasePlugin doesn't need to have all of it's attributes defined when it is first created. If you later add a FooPlugin child and use a migration to add columns to the base_plugins table to add attributes to FooPlugin, BasePlugin will be able to see those columns. ActiveRecord uses introspection to populate the database columns into the ActiveRecord object at startup, so BasePlugin.attribute_names will show you all columns, even if they are only used by the child type.
Yes you can implement what you describe, as ActiveRecord loads the schema after it connects to the database.
I have written about "best practices" using STI with Rails + ActiveRecord on another stackoverflow question here which might be helpful.
For your use case you will want to make sure plugins don't clash their columns, so you will probably need some kind of column naming scheme/prefix for each plugin.
I no longer use ActiveRecord with Rails, and prefer the more powerful and more performant data access layer Sequel. If you're not locked into ActiveRecord you may want to consider Sequel as an alternative as it supports both STI and also CTI (Class Table Inheritence) as well as the latest Postgres features like JSONB which may be better suited to your use case to keep plugins from clashing with each others columns, or simply just storing your plugin related data in a fully indexed JSON column.
I understand the fact that generating a model based on the DataBaseFirst method woill produce a collection of entitites on the ORM that are essentially mapped to the DB tables.
It is my understanding, that if you need properties from other entities or just dropdownlist fields, you can make a ViewModel and use that class as your model.
I have an AppDev course that I just finished and the author wrote something that if I understand it correctly, he is referring to change the ORM entities to fit what your ViewModels would look like, hence, no need for ViewModels. However, if you do this, and regenerate the ORM from the database, those new entities that you placed as "ViewModels" would be gone. If you changed the ORM to update the database, then your database structure in SQL Server would be "undone".
Please inform me if my understanding is correct that I simply need to use a ViewModel in a separate folder to gather specific classes and or properties in a superclass or a single class with the properties that I just need and use that as my model....
Here is the excerpt from the author:
EntityFramework is initially a one to one mapping of classes to tables, but you can create a model that better represents the entities in your application no matter how the data is stored in relational tables.
What I think the author may have been hinting at is the concept of complex models. Let's say, for instance, that in your Database you have a Customer Table and an Address Table. A one to one mapping would create 2 model items, one Customer class and one Address class. Using complex model mapping, you could instead create a single Customer class which contained the columns from both the Customer Table and the Address table. Thus, instead of Customer.Address.Street1 you could refer simply to Customer.Street1. This is only one of many cases where you could represent a conceptual model in code differently than the resulting data in storage. Check out the excellent blog series Inheritance with EF CodeFirst for examples of different mapping strategies, like Table Per Hierarchy (TPH), Table Per Type (TPT), Table Per Concrete Class (TPC). Note that even though these examples are CodeFirst, they still apply to Entity Framework even if the initial models are generated from a Database.
In general, if you use DatabaseFirst and then never modify the resulting entities, you will always have a class in code for each table in the database. However, the power of Enity Framework is that it allows you to more efficiently work with your entities by allowing these hybrid mappings, thus freeing you to think about your code without the extra burden of your classes having to abide by rigid SQL expectations.
Edit
When the Database-First or Model-First entities are generated, they are purposely generated as partial classes. You can create your own partials which extend the classes that are generated from Entity Framework without modifying the existing class files. If the models are re-generated, your partial classes will still be intact. Granted, using partials means that you have the Entity Framework default behaviors as well as your extended behaviors, but this isn't usually a huge issue.
Another option is that you can modify the TT files which Entity Framework uses to generate your models, ensuring that your models are always regenerated in the same state.
Ultimately, the main idea is that just because the default behavior of Entity Framework is to map the database to classes 1:1, there are other options and you are never forced into something that isn't efficient for your project.
I'm very new to AngularJS and MVVM in general and was looking for the best way to deal with displaying a model that also needs to display a couple of fields from child model objects on the web UI.
At the moment following basic tutorials my model object in my AngularJS controller reflects exactly my Rails model, and so I can't access fields from my child objects and just see their IDs from the foreign key columns in the database.
I'm wondering what the best convention is for dealing with this situation?
do I create a tableless model in Rails that contains only the fields needed by the presentation layer. Me defining what the presentation layer needs in Rails seems like it defeats the point of using MVVM.
do I create something on the AngularJS side that queries for the child objects using those exposed foreign key IDs? If so, how do I optimise it to avoid performing a request for each of my 50 objects in the table.
Yeah! You should define your api to contain all the data needed for a request. You don't need to create a tabless model for that.
If for example a fetch the json for a Post, Author and Comments json representation also should be there, or at least some part of it.
You might want to take a look at https://github.com/rails-api/active_model_serializers or https://github.com/nesquena/rabl
I want to aggregate data from different sources, Twitter lastfm and that sort. I just can't figure out how to store the data. Clearly in a database but I can't figure out how abstract to make the table to hold all this data without compromising the logical understanding of the data in each column.
I was wondering if anybody else had experience with this and now they tackled it in rails.
One option, if you want to stick with SQL, would be to have a Model/Table which contains fields common to every data source (title, url, summary) which is associated to other Models/Tables which contain the fields specific to individual data sources. The associations could be regular or polymorphic. And if you wanted to get in to some metaprogramming you could use method_missing to delegate method calls for fields not present in the 'common' Model to the associated models. This would work best with a polymorphic join. Psudeo-code:
class DataSource
belongs_to :data_source_extension, :polymorphic => true
def method_missing(method)
if data_source_extension.responds_to? method
data_source_extension.send(method)
else
super
end
end
end
The other option would be STI, so one table with all fields and a 'type' field which tells Rails which model the record should be wrapped in. This depends on how many different sources you have and how different they are from each other.
If the fields don't need to be searchable storing a Hash in a Text field works well. See Serialize and the attr_bucket gem.
Or if you want to trendy a NoSQL type database allows on-the-fly fields to be generated.
What you need is a document-oriented database (I recommend you MongoDB), and then having a set of adapters, one for each type of provider.
Document oriented database