Get a list of elements by their ID in entity framework

Question

How can I get all elements that are in another list by ID? I have List roles; I'd like to get all roles from the database that are in this list by their Id.

I'm using code-first.

I did this and it threw an error:

var roles = db.Roles.Where(r => user.Roles.Any(ur => ur.RoleId == r.RoleId));

RoleId is of type int.

Error:

Unable to create a constant value of type 'SampleMVC.Domain.Role'. Only primitive types ('such as Int32, String, and Guid') are supported in this context.

Solution 1

var listOfRoleId = user.Roles.Select(r => r.RoleId);
var roles = db.Roles.Where(r => listOfRoleId.Contains(r.RoleId));

Solution 2

Something like this should work if user.Roles is a list of ints:

var roles = db.Roles.Where(r => user.Roles.Contains(r.RoleId));

That turns it into a "SELECT WHERE IN (x, y, z...)" in SQL.

Solution 3

You can't combine a local list with remote data, then there is nothing for the db to read from since the data is elsewere (on your client).

I think there might be better solution to what you're trying to do;

It seems like you're trying to fetch all roles assigned to a specific user. If that's the case i would suggest a solution where you're passing the current user id to the database and fetch the roles assigned with a INNER JOIN.

Depending on your database it might look something like this (if you're connecting users with roles through a table called 'UserRoles')

var roles = db.UserRoles.Where(x => x.UserID == <insert id>).Select(x => x.Role)

(Of course you could also create a stored procedure returning a list of 'Role' if you like directly in your db and map it.)

Comments

[Shawn Mclean]

I cannot use r.RoleId because Roles is a class and RoleId is an int.

[Slauma]

Idea is good! You could insert a .Select(ur => ur.RoleId). between ...user.Roles... and ....Contains.... I'm curious if the query works then.

[moi_meme]

Should be, check the generated SQL, or try it in LinqPad

[Ladislav Mrnka]

If the user is loaded entity and if it has loaded Roles it will be one query.

[Jess]

This works fine for small data sets, but if the list of ID's is large, you will want to do it another way. stackoverflow.com/questions/1869753/…

[Amit Andharia]

Can this work with joins too? I've tried the same thing, and its giving me this error : Cannot compare elements of type 'System.Collections.Generic.List`1[[System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]'. Only primitive types, enumeration types and entity types are supported.

[Rolando Retana]

@AmitAndharia Did you solve that problem? I am getting the same error when using joins.

[Amit Andharia]

@RolandoPérez - Yes, use the Contains as shown in the answer above

[Shehroze Malik]

Works like a charm :)