Question

I’m writing a query using Linq to Entities and Entity Framework. My app uses two databases. Cross database is not an option due to client demands.

Lets say MyEntity resides in first database (so it is on one EF Model) and it references an entity in the second database SecondDBEntity (so it is on another EF Model), which has a composite key. Now, lets say I want to retrieve all MyEntity which references a set of SecondDBEntity (In our example, entities with key [1, 1], [1, 2] and [1, 3]).

The SQL I want to generate is this:

SELECT *
  FROM MyEntity
 WHERE (MyEntity.ForeignKeyOne = 1 AND MyEntity.ForeignKeyTwo = 1)
    OR (MyEntity.ForeignKeyOne = 1 AND MyEntity.ForeignKeyTwo = 2)
    OR (MyEntity.ForeignKeyOne = 1 AND MyEntity.ForeignKeyTwo = 3)

I have then tried the following code:

var setOfEntitiesToSearch = LetsAssumeThisIsAnIEnumerableOfSecondDBEntity;
return (from myEntity in DataContext.MyEntityList
       where setOfEntitiesToSearch.Any(entityToSearch => entityToSearch.KeyOne == myEntity.ForeignKeyOne && entityToSearch.KeyTwo == myEntity.ForeignKeyTwo)
      select myEntity).ToList();

This code compiles fine, but when I execute, it gives me the error:

"Unable to create a constant value of type 'SecondDBEntity'. Only primitive types or enumeration types are supported in this context."

My main problem is because entities are in different databases. I’m probably doing something silly, because this is a farely common query I’m trying to build. So, I believe I’m missing some feature in EF which will allow me to create this query. Maybe some CompositeKey structure? Or some way to mix those EF Models?

Thanks in advance.

Answer 1

Linq can’t translate the setOfEntitiesToSearch into a sql query. It can only use primative types. Note, an IEnumerable is not the same as an enum. Linq can use an enum because it can be translated to a value of type integer, but it can’t translate an IEnumerable into a primative type.

From what I can think of, you could go about this two different ways. (Note, I’m typing this code without compiling, so it may need debugged.)

1) Loop through setOfEntitiesToSearch and build a List like this:

List<MyEntity> results = new List<MyEntity>();
foreach (item in setOfEntitiesToSearch) {
    results.AddRange(
        from myEntity in DataContext.MyEntityList           
        where myEntity.ForeignKeyOne == item.KeyOne && myEntity.ForeignKeyTwo == item.KeyTwo
        select myEntity).ToList()
    );
}
return results;

2) Get all DataContext.MyEntityList into an in-memory list and query it.

var entityList = DataContext.MyEntityList.ToList()
return entityList.Where(myEntity => setOfEntitiesToSearch.Any(entityToSearch => entityToSearch.KeyOne == myEntity.ForeignKeyOne && entityToSearch.KeyTwo == myEntity.ForeignKeyTwo)).ToList()

Drawback to #1 – calling the db in a loop.

Drawback to #2 – entityList may be very large depending on the number of items DataContext.MyEntityList returns.