Can someone explane the following code for me?
public class StoreEditorViewModel
{
public List<Ticket> TotalView { get; set; }
public StoreEditorViewModel()
{
using (MvcTicketsEntities storeDB = new MvcTicketsEntities())
{
var temp = storeDB.Tickets.Include(x => x.Genres).Include(x => x.Artists).ToList();
TotalView = temp.ToList();
}
}
}
I don't understand the Inculde(x => x.genres) *genres is another table in my database. ( i use entity Framework)
The Include is telling EF to fetch the Genres records as part of this sql request, rather than making you call twice (once for Tickets and again for the Tickets Genres).
To quote Jon Galloway in the MVC Music Store example (your code looks very similar)
"We’ll take advantage of an Entity Framework feature that allows us to indicate other related entities we want loaded as well when the Genre object is retrieved. This feature is called Query Result Shaping, and enables us to reduce the number of times we need to access the database to retrieve all of the information we need. We want to pre-fetch the Albums for Genre we retrieve, so we’ll update our query to include from Genres.Include(“Albums”) to indicate that we want related albums as well. This is more efficient, since it will retrieve both our Genre and Album data in a single database request."
Related
I am new to entity framework and I am trying to get my head around it. I am used to writing stored procedures which have all the data I need on a example by example basis.
I am under the impression that I can get all values from a particular table including the foreign key values direct using entity framework without having to write a select query which joins the data.
I have the following in my controller
public ActionResult Patient()
{
using (var context = new WaysToWellnessDB())
{
var patients = context.Patients.ToList();
return View(patients);
}
}
In my view I have the following
#foreach (var item in Model)
{
<p>
#item.FirstName #item.Surname #item.Gender.GenderDesc
</p>
}
I have two tables, Patient and Gender, GenderId is a foreign key which I am trying to get the GenderDesc from that table.
I am getting the following message
The ObjectContext instance has been disposed and can no longer be used for operations that require a connection.
Can someone explain why I cannot access GenderDesc. It does work if I remove the using() around my context, but I don't really want to leave that open, is there a way to get this to work still having the using around?
Thanks in advance.
Correct, you have disposed of the context as it is within a using statement, so anything you try to access from then on will not be able to be lazy loaded. The disadvantage with lazy loading is that it will perform a query for the gender for every patient you are iterating over, which is handy, but bad! I would load the related table at query time using Include.
You'll need a new import:
using System.Data.Entity;
And then include the related table:
var patients = context.Patients.Include(p => p.Gender).ToList();
That will result in a query which will join to your "Gender" table and you should be able to output item.Gender.GenderDesc in your view.
What's the most preferred way to work with Entity Framework and DTOs?
Let's say that after mapping I have objects like:
Author
int id
sting name
List<Book> books
Book
int id
string name
Author author
int authorID
My DTOs
AuthorDTO
int id
sting name
BookDTO
int id
string name
int authorID
Since author can have a lot of books I don't want to retrieve all of them, when for example I'm only interested in authors.
But sometimes I might want to get few authors and filtered books or all books.
I could go with multiple queries AuthorDTO GetAuthor(int id) List<BookDTO> GetBooks(int authorID). But that means several accesses to database.
The ways I see it:
If I had in AuthorDTO field List<BookDTO> books the job could be done. But sometimes I would keep this list empty, if for example I listed only authors. And that means some unconsistency, mess and a lot of details to remember.
Return Tuple<AuthorDTO, List<BookDTO>> it might be a bit confusing.
Define new DTO.
AuthorAndBooksDTO
AuthorDTO author
List<BookDTO> books
The problem with sticking to a sinlge AuthorDTO and selectively filling the List is that you are now forced to keep track of where that DTO came from. Is the list of Books not hydrated, or does this Author simply have no books? Do I have to go back to my controller and call a different method to get a different state of the same DTO? This lacks clarity from the consumer's standpoint.
In my experience, I've leaned the way of more DTOs instead of trying to re-use a set of basic DTOs to represent multiple different sets of data. It requires a bit more "boilerplate", having to set up a whole bunch of similar DTOs and mappings between DTO and Entity, but in the end the specificity and clarity makes the codebase easier to read and manage.
I think some clarification of the issues involved will actually solve your confusion here.
First and most importantly, your entity classes are DTOs. In fact, that's all they are. They're classes that represent a table structure in your database so that data from queries Entity Framework makes can be mapped on to them. In other words, they are literally objects that transfer data. The failing of Microsoft and subsequently far too many MVC developers is to conflate them with big-M Models described by the MVC pattern.
As a result, it makes absolutely zero sense to use Entity Framework to return one or more instances of an entity and then map that to yet another DTO class before finally utilizing it in your code. All you're doing is creating a pointless level of abstraction that adds nothing to your application but yet another thing to maintain.
As far as relationships go, that's where Entity Framework's lazy/eager loading comes in. In order to take advantage of it, though, the property representing the relationship must follow a very specific convention:
public virtual ICollection<Book> Books { get; set; }
If you type it as something like List<Book>, Entity Framework will not touch the relationship at all. It will not ever load the related entities and it will not persist changes made to that property when saving the entity back to the database. The virtual keyword allows Entity Framework to dynamically subclass your entity and override the collection property to add the logic for lazy-loading. Without that, the related entities will only ever be loaded if you explicitly use Load from the EF API.
Assuming your property is defined in that way, then you gain a whole world of abilities. If you want all books belonging to the author you can just interact with author.Books directly (iterate, query, whatever). No queries are made until you do something that requires evaluation of the queryset. EF issues just-in-time queries depending on the information you're requesting from the database. If you do want to load all the related books at the same time you retrieve the author, you can just use Include with your query:
var author = db.Authors.Include(m => m.Books).SingleOrDefault(m => m.Id == id);
My first question would be to ask why you are creating DTO's in the first place? Is there a consumer on the other end that is using this data? Is it a screen? Are you building DTO's just to build DTO's?
Since you tagged the question as MVC i'm going to assume you are sending data to a view. You probably want a ViewModel. This ViewModel should contain all the data that is shown on the View that uses it. Then use entity framework to populate the view model. This may be done with a single query using projections or something complex.
So after all that blathering. I would say you want option 3.
Just like the others said, for clarity reasons, you should avoid creating "generic" DTO's for specific cases.
When you want to sometimes have authors and some of their books then model a DTO for that.
When you need only the authors then create another DTO that is more suited for that.
Or maybe you don't need DTOs, maybe a List containing their names is enough. Or maybe you could in fact use an anonymous type, like new { AuthorId = author.Id, AuthorName = author.Name }. It depends on the situation.
If you're using ASP.NET MVC the DTO you'll want is in fact a ViewModel that best represents your page.
Based on what you've described, you're view model could be something like this
public class BookViewModel{
public int Id {get;set;}
public string Name {get;set;}
}
public class AuthorViewModel{
public int Id {get;set;}
public string Name {get;set;}
public List<BookViewModel> Books {get;set;} = new List<BookViewModel>();
}
public class AuthorsViewModel
{
public List<AuthorViewModel> Authors {get;set;} = new List<AuthorViewModel>();
//add in this class other properties, like the filters used on the page...
public void Load(){
//here you can retrieve the data from your database.
//you could do like this:
//step 1: retrieve data from DB via EF
//step 2: fill in the Authors view models from the data at step 1
}
}
//and in your controller you're calling the Load method to fill you're viewmodel with data from db.
public class AuthorsController{
public ActionResult Index(){
AuthorsViewModel model = new AuthorsViewModel();
model.Load();
return View(model);
}
}
I have 2 classes, like the below.
They can have very large collections - a Website may have 2,000+ WebsitePages and vice-versa.
class WebsitePage
{
public int ID {get;set;}
public string Title {get;set;}
public List<Website> Websites {get;set;}
}
class Website
{
public int ID {get;set;}
public string Title {get;set;}
public List<WebsitePage> WebsitePages {get;set;}
}
I am having trouble removing a WebsitePage from a Website. Particularly when removing a WebsitePage from mutliple Websites.
For example, I might have code like this:
var pageToRemove = db.WebsitePages.FirstOrDefault();
var websites = db.Websites.Include(i => i.WebsitePages).ToList();
foreach(var website in websites)
{
website.WebsitePages.Remove(pageToRemove)
}
If each website Include() 2k pages, you can imagine it takes ages to load that second line.
But if I don't Include() the WebsitePages when fetching the Websites, there is no child collection loaded for me to delete from.
I have tried to just Include() the pages that I need to delete, but of course when saving that gives me an empty collection.
Is there a recommended or better way to approach this?
I am working with an existing MVC site and I would rather not have to create an entity class for the join table unless absolutely necessary.
No, you can't... normally.
A many-to-many relationship (with a hidden junction table) can only be affected by adding/removing items in the nested collections. And for this the collections must be loaded.
But there are some options.
Option 1.
Delete data from the junction table by raw SQL. Basically this looks like
context.Database.ExecuteSqlCommand(
"DELETE FROM WebsiteWebsitePage WHERE WebsiteID = x AND WebsitePageID = y"));
(not using parameters).
Option 2.
Include the junction into the class model, i.e. map the junction table to a class WebsiteWebsitePage. Both Website and WebsitePage will now have
public ICollection<WebsiteWebsitePage> WebsiteWebsitePages { get; set; }
and WebsiteWebsitePage will have reference properties to both Website and WebsitePage. Now you can manipulate the junctions directly through the class model.
I consider this the best option, because everything happens the standard way of working with entities with validations and tracking and all. Also, chances are that sooner or later you will need an explicit junction class because you're going to want to add more data to it.
Option 3.
The box of tricks.
I tried to do this by removing a stub entity from the collection. In your case: create a WebsitePage object with a valid primary key value and remove it from Website.WebsitePages without loading the collection. But EF doesn't notice the change because it isn't tracking Website.WebsitePages, and the item is not in the collection to begin with.
But this made me realize I had to make EF track a Website.WebsitePages collection with 1 item in it and then remove that item. I got this working by first building the Website item and then attaching it to a new context. I'll show the code I used (a standard Product - Category model) to prevent typos.
Product prd;
// Step 1: build an object with 1 item in its collection
Category cat = new Category { Id = 3 }; // Stub entity
using(var db = new ProdCatContext())
{
db.Configuration.LazyLoadingEnabled = false;
prd = db.Products.First();
prd.Categories.Add(cat);
}
// Step 2: attach to a new context and remove the category.
using(var db = new ProdCatContext())
{
db.Configuration.LazyLoadingEnabled = false;
db.Products.Attach(prd);
prd.Categories.Remove(cat);
db.SaveChanges(); // Deletes the junction record.
}
Lazy loading is disabled, otherwise the Categories would still be loaded when prd.Categories is addressed.
My interpretation of what happens here is: In the second step, EF not only starts tracking the product when you attach it, but also its associations, because it 'knows' you can't load these associations yourself in a many to many relationship. It doesn't do this, however, when you add the category in the first step.
I am playing around a bit with Raven and trying to figure out what the best way would be to model my objects for a twitter-like scenario. So far I have come up with a few options but not sure which one is the best.
public class User{
public string Id{get;set;}
public List<string> Following{get;set;}
public List<string> Followers{get;set;}
}
The User object is simple and straightforward, just an ID and a list of IDs for people I follow and people following me. The feed setup is where I need help, getting all posts from users that I am following.
Option 1 - The easy route
This searches for all posts of people I follow just based on their UserId.
public class Post{
public string UserId{get;set;}
public string Content{get;set;}
}
Index
public class Posts : AbstractIndexCreationTask<Post>{
public Posts(){
Map = results => from r in results
select new{
r.UserId
};
}
}
Querying
var posts = session.Query<Post,Posts>().Where(c=>c.UserId.In(peopleImFollowing));
This is the obvious route but it smells bad. The query results in a bunch of OR statements sent to Lucene. There is an upper limit of somewhere around 1024 that Raven will handle, so any one user couldn't follow more than 1000 people.
Option 2 - One post for each follower
public class Post{
public string UserId{get;set;}
public string RecipientId{get;set;}
public string Content{get;set;}
}
Adding a new post
foreach(string followerId in me.Followers){
session.Store(new Post{
UserId = me.UserId,
RecipientId = followerId,
Content = "foobar" });
}
This is simple to follow and easy to query but it seems like there would be way too many documents created... perhaps that doesn't matter though?
Option 3 - List of recipients
So far I like this the best.
public class Post{
public string UserId{get;set;}
public List<string> Recipients{get;set;}
public string Content{get;set;}
}
Index
public class Posts : AbstractIndexCreationTask<Post>{
public Posts(){
Map = results => from r in results
select new{
UserId = r.UserId,
Recipient = r.Recipients
}
}
}
Adding new post
session.Store(new Post{
UserId = me.Id,
Recipients = me.Followers,
Content = "foobar"
});
Querying
var posts = session.Query<Post,Posts>().Where(c=>c.Recipient == me.Id);
This seems like the best way but I have never worked with Lucene before. Would it be a problem for the index if someone has 10,000 followers? What if we want to post a message that goes to every single user? Perhaps there is another approach?
From my perspective, only option 1 really works and you will probably want to tune how RavenDB talks to lucene if you want to have support for following more than 1024 users.
Option 2 and Option 3 don't take into account that after you have followed new users you want older tweets of them to show up in your timeline. Likewise, you also want these tweets disappear from your timeline after you unfollowed them. If you want to implement this with one of those two approaches, you would need to duplicate all of their tweets on 'follow' operation and also delete them on 'unfollow'. This would make following/unfollowing a very expensive operation and it could also fail (what if the server that contains parts of the tweets isn't available the moment you're doing this?).
Option 2 also has the immensive disadvantage that it would produce literally tons of duplicate data. Think about famous users with millions of followers and thousands of posts. Then multiply this with thousands of famous users... not even twitter can handle such amounts of data.
Option 3 also has the problem that queries to the index get slow because every lucene document would have this 'recipient' field with perhaps millions of values. And you have trillions of documents... no, I'm not a lucene expert, but I don't think that works fast enough to display the timeline (even ignoring that you are not the only concurrent user that wants to display the timeline).
As I said above, I think that only option 1 works. Maybe someone else has a better approach. Good question btw.
I have an ASP.NET MVC website. In my backend I have a table called People with the following columns:
ID
Name
Age
Location
... (a number of other cols)
I have a generic web page that uses model binding to query this data. Here is my controller action:
public ActionResult GetData(FilterParams filterParams)
{
return View(_dataAccess.Retrieve(filterParams.Name, filterParams.Age, filterParams.location, . . .)
}
which maps onto something like this:
http://www.mysite.com/MyController/GetData?Name=Bill .. .
The dataAccess layer simply checks each parameter to see if its populated to add to the db where clause. This works great.
I now want to be able to store a user's filtered queries and I am trying to figure out the best way to store a specific filter. As some of the filters only have one param in the queryString while others have 10+ fields in the filter I can't figure out the most elegant way to storing this query "filter info" into my database.
Options I can think of are:
Have a complete replicate of the table (with some extra cols) but call it PeopleFilterQueries and populate in each record a FilterName and put the value of the filter in each of field (Name, etc)
Store a table with just FilterName and a string where I store the actual querystring Name=Bill&Location=NewYork. This way I won't have to keep adding new columns if the filters change or grow.
What is the best practice for this situation?
If the purpose is to save a list of recently used filters, I would serialise the complete FilterParams object into an XML field/column after the model binding has occurred. By saving it into a XML field you're also giving yourself the flexibility to use XQuery and DML should the need arise at a later date for more performance focused querying of the information.
public ActionResult GetData(FilterParams filterParams)
{
// Peform action to get the information from your data access layer here
var someData = _dataAccess.Retrieve(filterParams.Name, filterParams.Age, filterParams.location, . . .);
// Save the search that was used to retrieve later here
_dataAccess.SaveFilter(filterParams);
return View(someData);
}
And then in your DataAccess Class you'll want to have two Methods, one for saving and one for retrieving the filters:
public void SaveFilter(FilterParams filterParams){
var ser = new System.Xml.Serialization.XmlSerializer(typeof(FilterParams));
using (var stream = new StringWriter())
{
// serialise to the stream
ser.Serialize(stream, filterParams);
}
//Add new database entry here, with a serialised string created from the FilterParams obj
someDBClass.SaveFilterToDB(stream.ToString());
}
Then when you want to retrieve a saved filter, perhaps by Id:
public FilterParams GetFilter(int filterId){
//Get the XML blob from your database as a string
string filter = someDBClass.GetFilterAsString(filterId);
var ser = new System.Xml.Serialization.XmlSerializer(typeof(FilterParams));
using (var sr = new StringReader(filterParams))
{
return (FilterParams)ser.Deserialize(sr);
}
}
Remember that your FilterParams class must have a default (i.e. parameterless) constructor, and you can use the [XmlIgnore] attribute to prevent properties from being serialised into the database should you wish.
public class FilterParams{
public string Name {get;set;}
public string Age {get;set;}
[XmlIgnore]
public string PropertyYouDontWantToSerialise {get;set;}
}
Note: The SaveFilter returns Void and there is no error handling for brevity.
Rather than storing the querystring, I would serialize the FilterParams object as JSON/XML and store the result in your database.
Here's a JSON Serializer I regularly use:
using System.IO;
using System.Runtime.Serialization.Json;
using System.Text;
namespace Fabrik.Abstractions.Serialization
{
public class JsonSerializer : ISerializer<string>
{
public string Serialize<TObject>(TObject #object) {
var dc = new DataContractJsonSerializer(typeof(TObject));
using (var ms = new MemoryStream())
{
dc.WriteObject(ms, #object);
return Encoding.UTF8.GetString(ms.ToArray());
}
}
public TObject Deserialize<TObject>(string serialized) {
var dc = new DataContractJsonSerializer(typeof(TObject));
using (var ms = new MemoryStream(Encoding.UTF8.GetBytes(serialized)))
{
return (TObject)dc.ReadObject(ms);
}
}
}
}
You can then deserialize the object and pass it your data access code as per your example above.
You didn't mention about exact purpose of storing the filter.
If you insist to save filter into a database table, I would have following structure of the table.
FilterId
Field
FieldValue
An example table might be
FilterId Field FieldValue
1 Name Tom
1 Age 24
1 Location IL
3 Name Mike
...
The answer is much more simple than you are making it:
Essentially you should store the raw query in its own table and relate it to your People table. Don't bother storing individual filter options.
Decide on a value to store (2 options)
Store the URL Query String
This id be beneficial if you like open API-style apps, and want something you can pass nicely back and forth from the client to the server and re-use without transformation.
Serialize the Filter object as a string
This is a really nice approach if your purpose for storing these filters remains entirely server side, and you would like to keep the data closer to a class object.
Relate your People table to your Query Filters Table:
The best strategy here depends on what your intention and performance needs are. Some suggestions below:
Simple filtering (ex. 2-3 filters, 3-4 options each)
Use Many-To-Many because the number of combinations suggests that the same filter combos will be used lots of times by lots of people.
Complex filtering
Use One-To-Many as there are so many possible individual queries, it less likely they are to be reused often enough to make the extra-normalization and performance hit worth your while.
There are certainly other options but they would depend on more detailed nuances of your application. The suggestions above would work nicely if you are say, trying to keep track of "recent queries" for a user, or "user favorite" filtering options...
Personal opinion
Without knowing much more about your app, I would say (1) store the query string, and (2) use OTM related tables... if and when your app shows a need for further performance profiling or issues with refactoring filter params, then come back... but chances are, it wont.
GL.
In my opinion the best way to save the "Filter" is to have some kind of json text string with each of the "columns names"
So you will have something in the db like
Table Filters
FilterId = 5 ; FilterParams = {'age' : '>18' , ...
Json will provide a lot of capabilities, like the use of age as an array to have more than one filter to the same "column", etc.
Also json is some kind of standard, so you can use this "filters" with other db some day or to just "display" the filter or edit it in a web form. If you save the Query you will be attached to it.
Well, hope it helps!
Assuming that a nosql/object database such as Berkeley DB is out of the question, I would definitely go with option 1. Sooner or later you'll find the following requirements or others coming up:
Allow people to save their filters, label, tag, search and share them via bookmarks, tweets or whatever.
Change what a parameter means or what it does, which will require you to version your filters for backward compatibility.
Provide auto-complete functions over filters, possibly using a user's filter history to inform the auto-complete.
The above will be somewhat harder to satisfy if you do any kind of binary/string serialization where you'll need to parse the result and then process them.
If you can use a NoSql DB, then you'll get all the benefits of a sql store plus be able to model the 'arbitrary number of key/value pairs' very well.
Have thought about using Profiles. This is a build in mechanism to store user specific info. From your description of your problem its seems a fit.
Profiles In ASP.NET 2.0
I have to admit that M$ implementation is a bit dated but there is essentially nothing wrong with the approach. If you wanted to roll your own, there's quite a bit of good thinking in their API.