fixed issue 19 in a roundabout way.

source/MongoDB.Tests/Issues/Issue19.cs

@@ -0,0 +1,46 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+using NUnit.Framework;
+using MongoDB.Attributes;
+using MongoDB.Configuration;
+
+namespace MongoDB.Issues
+{
+    [TestFixture]
+    public class Issue19
+    {
+        private class Post
+        {
+            [MongoId]
+            public Guid Id { get; set; }
+
+            public DateTime LastAccess { get; set; }
+            public List<string> Tags { get; set; }
+        }
+
+        [Test]
+        public void Should_return_an_empty_list()
+        {
+            var SearchTags = new List<string> { "test" };
+            using (var mongo = new Mongo())
+            {
+                mongo.Connect();
+                var db = mongo.GetDatabase("posts");
+                var posts = db.GetCollection<Post>();
+                posts.Remove(x => true);
+                posts.Insert(new Post() { Tags = new List<string> { "test", "funny" } });
+
+                var result = SearchTags
+                    .Aggregate(posts.Linq(), (current, tag) => current.Where(x => x.Tags.Contains(tag)))
+                    .OrderByDescending(x => x.LastAccess)
+                    .ToList();
+
+                Assert.AreEqual(1, result.Count);
+            }
+        }
+
+    }
+}

source/MongoDB.Tests/MongoDB.Tests.csproj

@@ -94,6 +94,7 @@
     <Compile Include="IntegrationTests\SecondServer\TestPooledConnectionFactory.cs" />
     <Compile Include="IntegrationTests\Linq\LinqDomain.cs" />
     <Compile Include="IntegrationTests\Linq\MapReduceTests.cs" />
+    <Compile Include="Issues\Issue19.cs" />
     <Compile Include="UnitTests\Bson\BsonTestBase.cs" />
     <Compile Include="UnitTests\Bson\TestBsonBinary.cs" />
     <Compile Include="UnitTests\Bson\TestRoundTrips.cs" />

source/MongoDB/Linq/Translators/DocumentFormatter.cs

@@ -183,12 +183,24 @@ protected override Expression VisitMethodCall(MethodCallExpression m)
                 {
                     case "Contains":
                         field = m.Arguments[0] as FieldExpression;
-                        if (field == null)
-                            throw new InvalidQueryException(string.Format("The mongo field must be the argument in method {0}.", m.Method.Name));
-                        VisitPredicate(field, true);
-                        AddCondition("$in", EvaluateConstant<IEnumerable>(m.Object).OfType<object>().ToArray());
-                        PopConditionScope();
-                        return m;
+                        if (field != null)
+                        {
+                            VisitPredicate(field, true);
+                            AddCondition("$in", EvaluateConstant<IEnumerable>(m.Object).OfType<object>().ToArray());
+                            PopConditionScope();
+                            return m;
+                        }
+
+                        field = m.Object as FieldExpression;
+                        if (field != null)
+                        {
+                            VisitPredicate(m.Object, true);
+                            AddCondition(EvaluateConstant<object>(m.Arguments[0]));
+                            PopConditionScope();
+                            return m;
+                        }
+
+                        throw new InvalidQueryException(string.Format("The mongo field must be the argument in method {0}.", m.Method.Name));
                 }
             }
             else if (m.Method.DeclaringType == typeof(string))