Using a different query that always evaluates to false: { _id : { $type : -1 } }. This query is nice because it doesn't depend on the type of _id, because it works equally well at the top level or in an $elemMatch (whether the elements have an _id field or not), and as a bonus it uses the _id index when evaluated at the top level.

rstam · rstam · commit d6cf4f5ae1b4 · 2012-06-02T12:58:03.000-04:00
diff --git a/Driver/Linq/Translators/PredicateTranslator.cs b/Driver/Linq/Translators/PredicateTranslator.cs
@@ -207,10 +207,28 @@ private IMongoQuery BuildArrayLengthQuery(Expression variableExpression, Express
             return null;
         }
 
+        private IMongoQuery BuildBooleanQuery(bool value)
+        {
+            if (value)
+            {
+                return new QueryDocument(); // empty query matches all documents
+            }
+            else
+            {
+                return _queryBuilder.Type("_id", (BsonType)(-1)); // matches no documents (and uses _id index when used at top level)
+            }
+        }
+
         private IMongoQuery BuildBooleanQuery(Expression expression)
         {
             if (expression.Type == typeof(bool))
             {
+                var constantExpression = expression as ConstantExpression;
+                if (constantExpression != null)
+                {
+                    return BuildBooleanQuery((bool)constantExpression.Value);
+                }
+
                 var serializationInfo = _serializationInfoHelper.GetSerializationInfo(expression);
                 return new QueryDocument(serializationInfo.ElementName, true);
             }
@@ -336,9 +354,7 @@ private IMongoQuery BuildConstantQuery(ConstantExpression constantExpression)
             var value = constantExpression.Value;
             if (value != null && value.GetType() == typeof(bool))
             {
-                // simulate true or false with a tautology or a reverse tautology
-                // the particular reverse tautology chosen has the nice property that it uses the index to return no results quickly
-                return new QueryDocument("_id", new BsonDocument("$exists", (bool)value));
+                return BuildBooleanQuery((bool)value);
             }
 
             return null;
@@ -858,7 +874,7 @@ private IMongoQuery BuildStringIndexOfQuery(Expression variableExpression, Expre
                             if (index >= startIndex + count)
                             {
                                 // index is outside of the substring so no match is possible
-                                return _queryBuilder.NotExists("_id"); // matches no documents
+                                return BuildBooleanQuery(false);
                             }
                             else
                             {
@@ -892,7 +908,7 @@ private IMongoQuery BuildStringIndexOfQuery(Expression variableExpression, Expre
                             if (unescapedLength > startIndex + count - index)
                             {
                                 // substring isn't long enough to match
-                                return _queryBuilder.NotExists("_id"); // matches no documents
+                                return BuildBooleanQuery(false);
                             }
                             else
                             {
@@ -1099,12 +1115,12 @@ private IMongoQuery BuildStringCaseInsensitiveComparisonQuery(Expression variabl
                     if (operatorType == ExpressionType.Equal)
                     {
                         // == "mismatched case" matches no documents
-                        return _queryBuilder.NotExists("_id");
+                        return BuildBooleanQuery(false);
                     }
                     else
                     {
                         // != "mismatched case" matches all documents
-                        return new QueryDocument();
+                        return BuildBooleanQuery(true);
                     }
                 }
             }
@@ -1265,7 +1281,7 @@ private IMongoQuery BuildTypeComparisonQuery(Expression variableExpression, Expr
             var discriminator = discriminatorConvention.GetDiscriminator(nominalType, actualType);
             if (discriminator == null)
             {
-                return new QueryDocument(); // matches everything
+                return BuildBooleanQuery(true);
             }
 
             if (discriminator.IsBsonArray)
@@ -1296,7 +1312,7 @@ private IMongoQuery BuildTypeIsQuery(TypeBinaryExpression typeBinaryExpression)
             var discriminator = discriminatorConvention.GetDiscriminator(nominalType, actualType);
             if (discriminator == null)
             {
-                return new QueryDocument(); // matches everything
+                return BuildBooleanQuery(true);
             }
 
             if (discriminator.IsBsonArray)
diff --git a/DriverUnitTests/Linq/SelectQueryTests.cs b/DriverUnitTests/Linq/SelectQueryTests.cs
@@ -5630,7 +5630,7 @@ where c.S.ToLower() == "Abc"
             Assert.IsNull(selectQuery.Skip);
             Assert.IsNull(selectQuery.Take);
 
-            Assert.AreEqual("{ \"_id\" : { \"$exists\" : false } }", selectQuery.BuildQuery().ToJson());
+            Assert.AreEqual("{ \"_id\" : { \"$type\" : -1 } }", selectQuery.BuildQuery().ToJson());
             Assert.AreEqual(0, Consume(query));
         }
 
@@ -5722,7 +5722,7 @@ where c.S.ToUpper() == "abc"
             Assert.IsNull(selectQuery.Skip);
             Assert.IsNull(selectQuery.Take);
 
-            Assert.AreEqual("{ \"_id\" : { \"$exists\" : false } }", selectQuery.BuildQuery().ToJson());
+            Assert.AreEqual("{ \"_id\" : { \"$type\" : -1 } }", selectQuery.BuildQuery().ToJson());
             Assert.AreEqual(0, Consume(query));
         }
 
@@ -5768,7 +5768,7 @@ where c.S.ToUpper() == "Abc"
             Assert.IsNull(selectQuery.Skip);
             Assert.IsNull(selectQuery.Take);
 
-            Assert.AreEqual("{ \"_id\" : { \"$exists\" : false } }", selectQuery.BuildQuery().ToJson());
+            Assert.AreEqual("{ \"_id\" : { \"$type\" : -1 } }", selectQuery.BuildQuery().ToJson());
             Assert.AreEqual(0, Consume(query));
         }
 
diff --git a/DriverUnitTestsVB/Linq/SelectQueryTests.vb b/DriverUnitTestsVB/Linq/SelectQueryTests.vb
@@ -5331,7 +5331,7 @@ Namespace MongoDB.DriverUnitTests.Linq
             Assert.IsNull(selectQuery.Skip)
             Assert.IsNull(selectQuery.Take)
 
-            Assert.AreEqual("{ ""_id"" : { ""$exists"" : false } }", selectQuery.BuildQuery().ToJson())
+            Assert.AreEqual("{ ""_id"" : { ""$type"" : -1 } }", selectQuery.BuildQuery().ToJson())
             Assert.AreEqual(0, Consume(query))
         End Sub
 
@@ -5421,7 +5421,7 @@ Namespace MongoDB.DriverUnitTests.Linq
             Assert.IsNull(selectQuery.Skip)
             Assert.IsNull(selectQuery.Take)
 
-            Assert.AreEqual("{ ""_id"" : { ""$exists"" : false } }", selectQuery.BuildQuery().ToJson())
+            Assert.AreEqual("{ ""_id"" : { ""$type"" : -1 } }", selectQuery.BuildQuery().ToJson())
             Assert.AreEqual(0, Consume(query))
         End Sub
 
@@ -5467,7 +5467,7 @@ Namespace MongoDB.DriverUnitTests.Linq
             Assert.IsNull(selectQuery.Skip)
             Assert.IsNull(selectQuery.Take)
 
-            Assert.AreEqual("{ ""_id"" : { ""$exists"" : false } }", selectQuery.BuildQuery().ToJson())
+            Assert.AreEqual("{ ""_id"" : { ""$type"" : -1 } }", selectQuery.BuildQuery().ToJson())
             Assert.AreEqual(0, Consume(query))
         End Sub
 

Original file line number	Diff line number	Diff line change
`@@ -207,10 +207,28 @@ private IMongoQuery BuildArrayLengthQuery(Expression variableExpression, Express`
`207`	`207`	`return null;`
`208`	`208`	`}`
`209`	`209`
	`210`	`+ private IMongoQuery BuildBooleanQuery(bool value)`
	`211`	`+ {`
	`212`	`+ if (value)`
	`213`	`+ {`
	`214`	`+ return new QueryDocument(); // empty query matches all documents`
	`215`	`+ }`
	`216`	`+ else`
	`217`	`+ {`
	`218`	`+ return _queryBuilder.Type("_id", (BsonType)(-1)); // matches no documents (and uses _id index when used at top level)`
	`219`	`+ }`
	`220`	`+ }`
	`221`	`+`
`210`	`222`	`private IMongoQuery BuildBooleanQuery(Expression expression)`
`211`	`223`	`{`
`212`	`224`	`if (expression.Type == typeof(bool))`
`213`	`225`	`{`
	`226`	`+ var constantExpression = expression as ConstantExpression;`
	`227`	`+ if (constantExpression != null)`
	`228`	`+ {`
	`229`	`+ return BuildBooleanQuery((bool)constantExpression.Value);`
	`230`	`+ }`
	`231`	`+`
`214`	`232`	`var serializationInfo = _serializationInfoHelper.GetSerializationInfo(expression);`
`215`	`233`	`return new QueryDocument(serializationInfo.ElementName, true);`
`216`	`234`	`}`
`@@ -336,9 +354,7 @@ private IMongoQuery BuildConstantQuery(ConstantExpression constantExpression)`
`336`	`354`	`var value = constantExpression.Value;`
`337`	`355`	`if (value != null && value.GetType() == typeof(bool))`
`338`	`356`	`{`
`339`		`- // simulate true or false with a tautology or a reverse tautology`
`340`		`- // the particular reverse tautology chosen has the nice property that it uses the index to return no results quickly`
`341`		`- return new QueryDocument("_id", new BsonDocument("$exists", (bool)value));`
	`357`	`+ return BuildBooleanQuery((bool)value);`
`342`	`358`	`}`
`343`	`359`
`344`	`360`	`return null;`
`@@ -858,7 +874,7 @@ private IMongoQuery BuildStringIndexOfQuery(Expression variableExpression, Expre`
`858`	`874`	`if (index >= startIndex + count)`
`859`	`875`	`{`
`860`	`876`	`// index is outside of the substring so no match is possible`
`861`		`- return _queryBuilder.NotExists("_id"); // matches no documents`
	`877`	`+ return BuildBooleanQuery(false);`
`862`	`878`	`}`
`863`	`879`	`else`
`864`	`880`	`{`
`@@ -892,7 +908,7 @@ private IMongoQuery BuildStringIndexOfQuery(Expression variableExpression, Expre`
`892`	`908`	`if (unescapedLength > startIndex + count - index)`
`893`	`909`	`{`
`894`	`910`	`// substring isn't long enough to match`
`895`		`- return _queryBuilder.NotExists("_id"); // matches no documents`
	`911`	`+ return BuildBooleanQuery(false);`
`896`	`912`	`}`
`897`	`913`	`else`
`898`	`914`	`{`
`@@ -1099,12 +1115,12 @@ private IMongoQuery BuildStringCaseInsensitiveComparisonQuery(Expression variabl`
`1099`	`1115`	`if (operatorType == ExpressionType.Equal)`
`1100`	`1116`	`{`
`1101`	`1117`	`// == "mismatched case" matches no documents`
`1102`		`- return _queryBuilder.NotExists("_id");`
	`1118`	`+ return BuildBooleanQuery(false);`
`1103`	`1119`	`}`
`1104`	`1120`	`else`
`1105`	`1121`	`{`
`1106`	`1122`	`// != "mismatched case" matches all documents`
`1107`		`- return new QueryDocument();`
	`1123`	`+ return BuildBooleanQuery(true);`
`1108`	`1124`	`}`
`1109`	`1125`	`}`
`1110`	`1126`	`}`
`@@ -1265,7 +1281,7 @@ private IMongoQuery BuildTypeComparisonQuery(Expression variableExpression, Expr`
`1265`	`1281`	`var discriminator = discriminatorConvention.GetDiscriminator(nominalType, actualType);`
`1266`	`1282`	`if (discriminator == null)`
`1267`	`1283`	`{`
`1268`		`- return new QueryDocument(); // matches everything`
	`1284`	`+ return BuildBooleanQuery(true);`
`1269`	`1285`	`}`
`1270`	`1286`
`1271`	`1287`	`if (discriminator.IsBsonArray)`
`@@ -1296,7 +1312,7 @@ private IMongoQuery BuildTypeIsQuery(TypeBinaryExpression typeBinaryExpression)`
`1296`	`1312`	`var discriminator = discriminatorConvention.GetDiscriminator(nominalType, actualType);`
`1297`	`1313`	`if (discriminator == null)`
`1298`	`1314`	`{`
`1299`		`- return new QueryDocument(); // matches everything`
	`1315`	`+ return BuildBooleanQuery(true);`
`1300`	`1316`	`}`
`1301`	`1317`
`1302`	`1318`	`if (discriminator.IsBsonArray)`
Original file line number	Diff line number	Diff line change
`@@ -5630,7 +5630,7 @@ where c.S.ToLower() == "Abc"`
`5630`	`5630`	`Assert.IsNull(selectQuery.Skip);`
`5631`	`5631`	`Assert.IsNull(selectQuery.Take);`
`5632`	`5632`
`5633`		`- Assert.AreEqual("{ \"_id\" : { \"$exists\" : false } }", selectQuery.BuildQuery().ToJson());`
	`5633`	`+ Assert.AreEqual("{ \"_id\" : { \"$type\" : -1 } }", selectQuery.BuildQuery().ToJson());`
`5634`	`5634`	`Assert.AreEqual(0, Consume(query));`
`5635`	`5635`	`}`
`5636`	`5636`
`@@ -5722,7 +5722,7 @@ where c.S.ToUpper() == "abc"`
`5722`	`5722`	`Assert.IsNull(selectQuery.Skip);`
`5723`	`5723`	`Assert.IsNull(selectQuery.Take);`
`5724`	`5724`
`5725`		`- Assert.AreEqual("{ \"_id\" : { \"$exists\" : false } }", selectQuery.BuildQuery().ToJson());`
	`5725`	`+ Assert.AreEqual("{ \"_id\" : { \"$type\" : -1 } }", selectQuery.BuildQuery().ToJson());`
`5726`	`5726`	`Assert.AreEqual(0, Consume(query));`
`5727`	`5727`	`}`
`5728`	`5728`
`@@ -5768,7 +5768,7 @@ where c.S.ToUpper() == "Abc"`
`5768`	`5768`	`Assert.IsNull(selectQuery.Skip);`
`5769`	`5769`	`Assert.IsNull(selectQuery.Take);`
`5770`	`5770`
`5771`		`- Assert.AreEqual("{ \"_id\" : { \"$exists\" : false } }", selectQuery.BuildQuery().ToJson());`
	`5771`	`+ Assert.AreEqual("{ \"_id\" : { \"$type\" : -1 } }", selectQuery.BuildQuery().ToJson());`
`5772`	`5772`	`Assert.AreEqual(0, Consume(query));`
`5773`	`5773`	`}`
`5774`	`5774`