Why does EF generate this sql when querying reference property

When using the AdventureWorks database and issuing this query:

   AdventureWorksEntities entities = new AdventureWorksEntities();
   entities.Contacts.Where(x => x.FirstName == "A" 
                               || x.FirstName == "B" 
                               || x.FirstName == "C")
                     .ToList();

Will be translated to this sql, which is the best it can:

//ommited for brevity
FROM [Person].[Contact] AS [Extent1]
WHERE [Extent1].[FirstName] IN (N'A',N'B',N'C')

However when I issue this query:

entities.Employee.Where(x => x.Contact.FirstName == "A" 
                             || x.Contact.FirstName == "B" 
                             || x.Contact.FirstName == "C")
                .ToList();

I get this SQL:

//ommited for brevity
FROM   [HumanResources].[Employee] AS [Extent1]
INNER JOIN [Person].[Contact] AS [Extent2] ON [Extent1].[ContactID] = [Extent2].[ContactID]
LEFT OUTER JOIN [Person].[Contact] AS [Extent3] ON [Extent1].[ContactID] = [Extent3].[ContactID]
WHERE [Extent2].[FirstName] = N'A' OR [Exten开发者_StackOverflow社区t3].[FirstName] IN (N'B',N'C')

Why am I getting an inner and outer join and is EF splitting the where across both of them?

Of Note using contains creates the same SQL:

var names = new List<string>{"A", "B", "C"};
entities.Employee.Where(x => names.Contains(x.Contact.FirstName)).ToList();

EDIT: So it appears to be a EF bug, I've accepted the answer which provided a work around

EDIT: Opened connect issue, it's located here

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Write a Stored Procedure which accepts a TVP as input parameter and let EF materialize the results from the SP :)

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Looks like a bug in EF.

I think that the problems is related to you checking against a field in a sub table. EF does not see that all the OR clauses refer to the same table and the same field in that that table. Therefore, it creates that complicated join.

Try rewriting it to use CONTAINS instead of OR.

Using contains() in LINQ to SQL