CSHARP-1702: adding support for field projection into Distinct operation across a Where call.

Author: craiggwilson

src/MongoDB.Driver/Linq/Processors/Pipeline/MethodCallBinders/DistinctBinder.cs

@@ -45,6 +45,10 @@ public Expression Bind(PipelineExpression pipeline, PipelineBindingContext bindi
                     keySelector = fieldAsDocumentExpression.Expression;
                 }
             }
+            else if (pipeline.Projector is FieldExpression)
+            {
+                keySelector = pipeline.Projector;
+            }
             else
             {
                 var currentProjector = (ISerializationExpression)pipeline.Projector;

tests/MongoDB.Driver.Tests/Linq/MongoQueryableTests.cs

@@ -179,7 +179,7 @@ public async Task CountAsync_with_no_matches()
         }
 
         [SkippableFact]
-        public void Distinct_followed_by_where()
+        public void Distinct_document_followed_by_where()
         {
             RequireServer.Where(minimumVersion: "2.6.0");
             var query = CreateQuery()
@@ -192,6 +192,68 @@ public void Distinct_followed_by_where()
                 "{ $match: { '_id.A': 'Awesome' } }");
         }
 
+        [SkippableFact]
+        public void Distinct_document_preceded_by_select_where()
+        {
+            RequireServer.Where(minimumVersion: "2.6.0");
+            var query = CreateQuery()
+                .Select(x => new { x.A, x.B })
+                .Where(x => x.A == "Awesome")
+                .Distinct();
+
+            Assert(query,
+                1,
+                "{ $project: { 'A': '$A', 'B': '$B', '_id': 0 } }",
+                "{ $match: { 'A': 'Awesome' } }",
+                "{ $group: { '_id': '$$ROOT' } }");
+        }
+
+        [SkippableFact]
+        public void Distinct_document_preceded_by_where_select()
+        {
+            RequireServer.Where(minimumVersion: "2.6.0");
+            var query = CreateQuery()
+                .Where(x => x.A == "Awesome")
+                .Select(x => new { x.A, x.B })
+                .Distinct();
+
+            Assert(query,
+                1,
+                "{ $match: { 'A': 'Awesome' } }",
+                "{ $group: { '_id': { 'A': '$A', 'B': '$B' } } }");
+        }
+
+        [SkippableFact]
+        public void Distinct_field_preceded_by_where_select()
+        {
+            RequireServer.Where(minimumVersion: "2.6.0");
+            var query = CreateQuery()
+                .Where(x => x.A == "Awesome")
+                .Select(x => x.A)
+                .Distinct();
+
+            Assert(query,
+                1,
+                "{ $match: { 'A': 'Awesome' } }",
+                "{ $group: { '_id': '$A' } }");
+        }
+
+        [SkippableFact]
+        public void Distinct_field_preceded_by_select_where()
+        {
+            RequireServer.Where(minimumVersion: "2.6.0");
+            var query = CreateQuery()
+                .Select(x => x.A)
+                .Where(x => x == "Awesome")
+                .Distinct();
+
+            Assert(query,
+                1,
+                "{ $project: { 'A': '$A', '_id': 0 } }",
+                "{ $match: { 'A': 'Awesome' } }",
+                "{ $group: { '_id': '$A' } }");
+        }
+
         [Fact]
         public void First()
         {