I am working on an ASP.NET MVC 4 web application. I am using Entity Framework as the data access layer, using database first approach (.edmx file).
Currently I have a problem in join tables that are defined inside two different databases (i.e. I have two .edmx files).
For example if I want to join tables I am performing the following query:-
public ActionResult AutoComplete(string term)
{
var tech = repository.AllFindTechnolog(term).Take(100);//Call to the first database
var resources = repository.GetResources(tech.Select(a => a.IT360ID.Value).ToArray(), false);//call to the second database
var query = from techItems in tech
join resourcesItems in resources
on techItems.IT360ID.Value equals resourcesItems.RESOURCEID // join based on db2ID
orderby techItems.PartialTag
select new //code goes here
return Json(query, JsonRequestBehavior.AllowGet);
}
I will have two separate calls to the database, and a join inside the application server, which is not the best performance-oriented solution. Ideally the joins will happen completely inside the database engine.
I know that a stored procedure will allow me to join tables from different databases purely on the server, but I do not want to use SP because it will make my code less maintainable and less testable.
So I am searching for a solution where I can do the join using entity framework and to result in a single database join?
If you want to do it with a single database call you will have to create a View in the database that joins the 2 tables from separate db's. Once the view is created you can add it to EF as a single object, which you can manipulate further and Query off of. The view will basically be a table and it will be easily maintable and easy to bind to a strongly typed model
Another way ,similiar like you have posted, you can query separate .edmx files and then join them.
Yes, there is 2 calls to the database but it shouldn't be that expensive and probably won't notice a difference.
using(var db = new MyEntities())
using (var db2 = new MyEntities2())
{
var one = db.Table1.AsEnumerable();
var two = db2.Table2.AsEnumerable();
var result = from o in one
join t in two on o.Id equals t.Id
// blah blah
}
#CSharper's answer is close. As #Oliver mentioned in the comments, IEnumerable loads the table into application memory, leading to crashes if you have a large database.
The solution is to use IQueryable, which can be called with LINQ - this produces SQL which is much faster.
// This is a generic method, modify to your needs
public ActionResult Details(int? id)
var one = db.Table1.AsQueryable();
var two = db2.Table2.AsQueryable();
// since you're using MVC EF, I assume you want to put this in a viewmodel
// (in this case ObjectCombined)
// assume "id" is passed as parameter
Object1 result1 = (from o in one where one.id == id select o).Single();
Object2 result2 = (from t in two where t.id == o.id select t).Single();
ObjectCombined result = new ObjectCombined(result1, result2);
return View(result);
}
Might I suggest that you look into using a synonym in your database. For instance, you can create a synonym to the resources table in the database that your tech table is located. This will ensure that you will not need to maintain 2 EDMX files. Instead you can have a single EDMX file and you can simply join your tech table to the synonym of the resource table and voila - you are on your way.
UPDATE: Please note that if you are using synonyms there is an extra bit of work you will need to do to the EDMX file to get it working in Entity Framework. Here is a blog post that was put out by a programmer who got it to work. Here is the original stackoverflow question she asked.
HAPPY CODING!!! :)
you can create a view or a stored procedure, your sql statement can then make cross db query just make sure your credentials can DML or DDL on both db. otherwise try the nested using entities that will make sure you will not get the linq bug when you dont declare the db entity inside a using statement.
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.
Is there actually any official strategy for creating and using views with EF6.1 Code First? I can reverse the database into Code First and get tables not views. I can import the database into Model First and get views and then do Generate DB from Model and my edmx is modified and the views are converted to tables. MSFT seems to have all but abandoned Model First, and the new Update 2 seems to have reverse engineering for Code First so I feel forced to convert to Code First but has the EF team given support for any reasonable approach to using views or stored Procedures in Code First? After all CF is supposed to create the DB but what - are you no longer supposed to use either of these SQL Server features in a .NET EF application?
For starters you can use views and stored procedures that are already created in the database. For views you don't have to do any mapping if you create a code like this:
public TestContext : DbContext
{
public DbSet<User> AdminUsers { get; set; }
}
And you have a view in the database named dbo.AdminUsers and it can be mapped to the class User (contains all required properties with the same name as the property).
For stored procedures you can use the SqlQuery function either through the Database property of the DbContext such as:
var userActivityReport = context.Database.SqlQuery<UserActivityReport>(
"dbo.GetUserActivityReport #p0, #p1", startDate, endDate);
Or through the SqlQuery function of the DbSet class:
var newUsers = context.Users.SqlQuery("dbo.GetNewUsers #p0", count);
If you want to create, modify or delete views and stored procedures via Entity Framework you can either use custom migration operations see http://dolinkamark.wordpress.com/2014/05/03/creating-a-custom-migration-operation-in-entity-framework/
For event better integration you can use the Public mapping API with a library provided by a community member: https://codefirstfunctions.codeplex.com/
Getting an error client side with breeze: "Cannot call method 'map' of undefined" when trying to pull over some data. The difference between this action and one that works is that this action is calling a stored procedure and returning ObjectResult<T> instead of DbSet<T>.
Might this be why I get an error? Using Chrome Developer tools, I do see that the breeze controller is returning json data.
I have created a complex model type in the edmx for mapping the rows returned from the stored procedure.
The action in the breeze controller has a return type of IEnumerable<T>.
I experienced the same error when using an EF complex type. A workaround was to create a view in my database instead of using a complex type, set the stored procedure to return a type of the new view which had a primary key and then it worked. It would seem that breeze requires entities to have a primary key defined.
Hm... not quite sure what is happening, so just guessing here, but try adding an AsQueryable() to the result returned, and changing the result type to a IQueryable.
We don't have any stored proc tests for breeze yet, but this is impetus for me to add some :)
I had the very same issue, but thank God I figured out a solution. Instead of using a stored procedure, you should use a view, as Breeze recognizes views as DbSet<T>, just like tables. Say you have a SQL server table that contains two tables Customers and Orders.
Customers (**CustomerId**, FirstName, LastName)
Orders (OrderId, #CustomerId, OrderDate, OrderTotal)
Now, say you want a query that returns orders by CustomerId. Usually, you would do that in a stored procedure, but as I said, you need to use a view instead. So the query will look like this in the view.
Select o.OrderId, c.CustomerId, o.OrderDate, o.OrderTotal
from dbo.Orders o inner join dbo.Customers c on c.CustomerId = o.CustomerId
Notice there is no filtering (where ...). So:
i. Create a [general] view that includes the filtering key(s) and name it, say, OrdersByCustomers
ii. Add the OrdersByCustomers view to the entity model in your VS project
iii. Add the entity to the Breeze controller, as such:
public IQueryable<OrdersByCustomers> OrdersByCustomerId(int id)
{
return _contextProvider.Context.OrdersByCustomers
.Where(r => r.CustomerId == id);
}
Notice the .Where(r => r.CustomerId == id) filter. We could do it in the data service file, but because we want the user to see only his personal data, we need to filter from the server so it only returns his data.
iv. Now, that the entity is set in the controller, you may invoke it in the data service file, as such:
var getOrdersByCustomerId = function(orderObservable, id)
{
var query = breeze.EntityQuery.from('OrdersByCustomerId')
.WithParameters({ CustomerId: id });
return manager.executeQuery(query)
.then(function(data) {
if (orderObservable) orderObservable(data.results);
}
.fail(function(e) {
logError('Retrieve Data Failed');
}
}
v. You probably know what to do next from here.
Hope it helps.
Ok, so let me explain a little of what I am trying to do.
I have a table called WebsitePage that contains all of the pages on my site with some information about them. Then, I have a Customer table for all of my customers. Then I have another table called CustomerWebsitePage that stores customer values for some of the columns in the WebsitePage table.
So, using the entity framework, I imported these three tables. What I want to be able to do is return a strongly typed WebsitePage list that has any values from CustomerWebsitePage if there are any values for it. So, for example, say one of my customers added a CustomerWebsitePageName for one of my website pages. I want to return a list of WebsitePages that contains the CustomerWebsitePageName instead of the WebsitePage Name in that case. But the original WebsitePage Name for everything else since it wasn't overridden.
The kicker here is that my WebsitePage table has a foreign key to itself for a Parent/Child relationship. So, I also want to return the child WebsitePages at the same time. I tried using a function import to get what I wanted, but then of course I lost the ChildPages.
I have tried just about everything to get what I want using the Entity framework and LINQ. But so far almost everything I try ends up with an exception being thrown. Here are a few:
The EntityCollection has already been initialized
The entity or complex type 'MyEntityModel.WebsitePage' cannot be constructed in a LINQ to Entities query.
I have one idea of how I can get around all this, and that would be to duplicate the ParentPageID into my WebsitePage table, but this really seems to violate a lot of principles and I really just don't want to add the maintenance headache related to this.
Anyone have any ideas how to accomplish this type of thing?
A simple DB diagram. http://images.tehone.com/screenshots/2009-08-17_013009.png
The object that you need to return is a CustomerWebsitePage and not a WebsitePage. The reason is that whatever object you return must know about the customer since the property will need it to determine which field (CustomerPage or WebsitePage) to use.
Considering that, you can have a function CustomerWebsitePage.GetAllPagesForCustomer(Customer c) that would return an enumeration of pages. However, to achieve the functionality you are looking for, you must implement some read-through properties in the CustomerWebsitePage. Let's take the example of Name. Here would be how to implement it in CustomerWebsitePage:
public string Name
{
get{ if( String.IsNullOrEmpty(CustomerWebsitePageName) )
return WebsitePage.Name;
return CustomerWebsitePageName; }
}
For the Children pages, you could have a property:
public IEnumerable<CustomerWebPage> Children
{
get
{
return WebsitePage.Children.Select( it => it.Customer.CustomerID == this.CustomerID ); }
}
Note that with this setup you couldn't run EF Linq queries on these new fields (because they exist only in the objects themselves, not in the database mapping). You can pepper the code with Load() if you want to seamlessly load all the children, but it will cost you in performance. To ensure that all the children are loaded, try the following loading function:
IEnumerable<CusomterWebPage> GetAllPagesForCustomer(Customer c)
{
return Context.CustomerWebPageSet.Include("WebsitePage").Where( it => it.Customer.CustomerID == c.CustomerID );
}
I'm using LINQ to model my database, but I'm writing it by hand (Steve Sanderson recommends this in his ASP.NET MVC book). What I need to know is what's happening when you create an EntityRef, and how it's referenced. If I create my queries manually (without using LINQ), but use LINQ to model it, and I bring back just the ID of something, then reference the actual table column using EntityRef in the view, does it do the join, or does it re-query the database for that bit of information?
To clear things up, if I have this in my repository:
public IEnumerable<MyTable> ListMyTable(int? myColumnVar)
{
string query = "SELECT * FROM MyTable WHERE MyColumn = {0}";
return this.ExecuteQuery<MyTable>(query, myColumnVar);
}
and then in my controller I do this:
IEnumerable<MyTable> mytables = _contractsControlService.ListMyTable(1);
return View(mytables);
then in my view I do things like
<%=tbl.Ref.MyColumn %>
I'm referencing something set out by the LINQ model, but isn't actually in the table output. How does it get that data?
To clear things up further, we're using systems which require ultimate speed, so the LINQ-to-SQL is too slow for us, hence why we're using direct queries in our repository. I wouldn't mind using this EntityRef business if only I knew what was happening underneath.
You have most likely used the Object Relational designer to create an entity class for the Ref entity and an association between the MyTable and the Ref entity. You can also do that manually by creating the appropriate entity classes and using attributes to map the classes to the database schema. You can read the details in How to: Customize Entity Classes by Using the Code Editor (LINQ to SQL) on MSDN.
In your generated (or handwritten) code you will find some attributes:
[Table(Name="dbo.Ref")]
public partial class Ref : INotifyPropertyChanging, INotifyPropertyChanged
and
public partial class MyTable: INotifyPropertyChanging, INotifyPropertyChanged
{
[Association(Name="Ref_MyTable", Storage="_Ref", ThisKey="RefId",
OtherKey="Id", IsForeignKey=true)]
public Ref Ref
{
get
...
The entity classes combined with the attributes enables the Linq-to-Sql framework to retrieve entity classes directly from the database.
If you want to monitor the SQL genereated by Linq-to-Sql you can assign the DataContext.Log property:
context.Log = Console.Out;
In your example, navigating from MyTable to Ref, probably generates SQL along these lines:
SELECT Id, Field1, Field2, Field3
FROM Ref
WHERE Id = #RefId