LINQ 到实体 - 带有条件 where 子句的左外连接

Question

I have this project set up with EF4 and I'm using LINQ to Entities to form queries.

I'm having some trouble with a query that involves lots of conditional where clauses and several left outer joins.

I have partially solved the conditional where clauses with the following extension method (which I found here ).

public static IQueryable<TSource> WhereIf<TSource>(this IQueryable<TSource> source, bool condition, Expression<Func<TSource, bool>> predicate)
    {
        if (condition)
            return source.Where(predicate);
        else
            return source;
    }

The problem I have now is that LINQ to Entities doesn't recognize the extension method when I use it on the joined tables.

Here's a part of the query:

from p in context.Entities.OfType<Patient>()
    .WhereIf(!string.IsNullOrEmpty(name), p => p.Name.Contains(name))
from c in context.Contacts
    .Where(cp => cp.EntityID == p.EntityId).DefaultIfEmpty()
    .WhereIf(timestamp != null, c => c.Timestamp > timestamp)

I say partially solved because it works just fine the first time (patient name) in this query, but the second time (timestamp) it gives me this error:

LINQ to Entities does not recognize the method 'System.Linq.IQueryable`1[x.Contact] WhereIf[Contact](System.Linq.IQueryable`1[x.Contact], Boolean, System.Linq.Expressions.Expression`1[System.Func`2[x.Contact,System.Boolean]])' method, and this method cannot be translated into a store expression.

So, I'm wondering if anyone know how I can fix this?

Answer 1

The problem is that EF is unable to recognize WhereIf inside a SelectMany . Rewriting your query into method syntax yields something like this (query is incomplete):

context.Entities.OfType<Patient>()
  .WhereIf(!string.IsNullOrEmpty(name), p => p.Name.Contains(name))
  .SelectMany(
    p => context.Contacts
      .Where(cp => cp.EntityID == p.EntityId).DefaultIfEmpty()
      .WhereIf(timestamp != null, c => c.Timestamp > timestamp),
    (p, c) => new { p, c }
  )

The problem is the last WhereIf . You should be able to move it outside the SelectMany :

context.Entities.OfType<Patient>()
  .WhereIf(!string.IsNullOrEmpty(name), p => p.Name.Contains(name))
  .SelectMany(
    p => context.Contacts
      .Where(cp => cp.EntityID == p.EntityId).DefaultIfEmpty(),
    (p, c) => new { p, c }
  )
  .WhereIf(timestamp != null, x => x.c.Timestamp > timestamp)

You can inspect the resulting SQL to see if you get what you actually want by casting the query into ObjectQuery and calling the method ToTraceString :

var query = context.Entities.OfType<Patient>() ...
Console.WriteLine(((ObjectQuery) query).ToTraceString());

Answer 2

Why not do Where(timestamp == null || c.Timestamp > timestamp) ?