CSHARP-5473: Provide API to turn LINQ expression into MQL.

src/MongoDB.Driver/Linq/IMongoQueryProvider.cs

@@ -40,6 +40,15 @@ public interface IMongoQueryProvider : IQueryProvider
         /// <param name="cancellationToken">The cancellation token.</param>
         /// <returns>The value that results from executing the specified query.</returns>
         Task<TResult> ExecuteAsync<TResult>(Expression expression, CancellationToken cancellationToken = default(CancellationToken));
+
+        /// <summary>
+        /// Translates an Expression to MQL.
+        /// </summary>
+        /// <param name="expression">The expression.</param>
+        /// <param name="outputSerializer">The output serializer.</param>
+        /// <typeparam name="TResult">The type of the result.</typeparam>
+        /// <returns>An array of MQL pipeline stages represented as BsonDocuments.</returns>
+        BsonDocument[] Translate<TResult>(Expression expression, out IBsonSerializer<TResult> outputSerializer);
     }
 
     /// <summary>

src/MongoDB.Driver/Linq/Linq3Implementation/MongoQueryProvider.cs

@@ -55,6 +55,7 @@ protected MongoQueryProvider(
         public abstract TResult Execute<TResult>(Expression expression);
         public abstract Task<TResult> ExecuteAsync<TResult>(Expression expression, CancellationToken cancellationToken);
         public abstract ExpressionTranslationOptions GetTranslationOptions();
+        public abstract BsonDocument[] Translate<TResult>(Expression expression, out IBsonSerializer<TResult> outputSerializer);
     }
 
     internal sealed class MongoQueryProvider<TDocument> : MongoQueryProvider
@@ -152,5 +153,14 @@ public override ExpressionTranslationOptions GetTranslationOptions()
             var database = _database ?? _collection?.Database;
             return translationOptions.AddMissingOptionsFrom(database?.Client.Settings.TranslationOptions);
         }
+
+        public override BsonDocument[] Translate<TResult>(Expression expression, out IBsonSerializer<TResult> outputSerializer)
+        {
+            var translationOptions = GetTranslationOptions();
+            var executableQuery = ExpressionToExecutableQueryTranslator.Translate<TDocument, TResult>(provider: this, expression, translationOptions);
+            var stages = executableQuery.Pipeline.Ast.Stages;
+            outputSerializer = (IBsonSerializer<TResult>)executableQuery.Pipeline.OutputSerializer;
+            return stages.Select(s => s.Render().AsBsonDocument).ToArray();
+        }
     }
 }

tests/MongoDB.Driver.Tests/Linq/Linq3Implementation/Jira/CSharp5473Tests.cs

@@ -0,0 +1,57 @@
+/* Copyright 2010-present MongoDB Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+using System.Linq;
+using FluentAssertions;
+using MongoDB.Driver.Linq;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3Implementation.Jira
+{
+    public class CSharp5473Tests : Linq3IntegrationTest
+    {
+        [Fact]
+        public void Translate_expression_should_work()
+        {
+            var collection = GetCollection();
+            var queryable = collection.AsQueryable()
+                .Select(x => x.X + 1);
+            var expression = queryable.Expression; // queryable was just used as an easy way to create the expression and the provider
+            var provider = (IMongoQueryProvider)queryable.Provider;
+
+            var stages = provider.Translate<int>(expression, out var outputSerializer);
+            AssertStages(stages, "{ $project : { _v : { $add : ['$X', 1] }, _id : 0 } }");
+
+            var pipeline = new BsonDocumentStagePipelineDefinition<C, int>(stages, outputSerializer);
+            var result = collection.Aggregate(pipeline).Single();
+            result.Should().Be(2);
+        }
+
+        private IMongoCollection<C> GetCollection()
+        {
+            var collection = GetCollection<C>("test");
+            CreateCollection(
+                collection,
+                new C { Id = 1, X = 1 });
+            return collection;
+        }
+
+        private class C
+        {
+            public int Id { get; set; }
+            public int X { get; set; }
+        }
+    }
+}