CSHARP-5478: Add support for $rankFusion aggregation stage

src/MongoDB.Driver/AggregateFluent.cs

@@ -257,6 +257,28 @@ public override IAggregateFluent<TNewResult> Project<TNewResult>(ProjectionDefin
             return WithPipeline(_pipeline.Project(projection));
         }
 
+        public override IAggregateFluent<TNewResult> RankFusion<TNewResult>(
+            Dictionary<string, PipelineDefinition<TResult, TNewResult>> pipelines,
+            Dictionary<string, double> weights = null,
+            RankFusionOptions<TNewResult> options = null)
+        {
+            return WithPipeline(_pipeline.RankFusion(pipelines, weights, options));
+        }
+
+        public override IAggregateFluent<TNewResult> RankFusion<TNewResult>(
+            PipelineDefinition<TResult, TNewResult>[] pipelines,
+            RankFusionOptions<TNewResult> options = null)
+        {
+            return WithPipeline(_pipeline.RankFusion(pipelines, options));
+        }
+
+        public override IAggregateFluent<TNewResult> RankFusion<TNewResult>(
+            (PipelineDefinition<TResult, TNewResult>, double?)[] pipelinesWithWeights,
+            RankFusionOptions<TNewResult> options = null)
+        {
+            return WithPipeline(_pipeline.RankFusion(pipelinesWithWeights, options));
+        }
+
         public override IAggregateFluent<TNewResult> ReplaceRoot<TNewResult>(AggregateExpressionDefinition<TResult, TNewResult> newRoot)
         {
             return WithPipeline(_pipeline.ReplaceRoot(newRoot));

src/MongoDB.Driver/AggregateFluentBase.cs

@@ -225,6 +225,29 @@ public virtual Task<IAsyncCursor<TResult>> OutAsync(IMongoCollection<TResult> ou
         /// <inheritdoc />
         public abstract IAggregateFluent<TNewResult> Project<TNewResult>(ProjectionDefinition<TResult, TNewResult> projection);
 
+        /// <inheritdoc />
+        public virtual IAggregateFluent<TNewResult> RankFusion<TNewResult>(
+            Dictionary<string, PipelineDefinition<TResult, TNewResult>> pipelines,
+            Dictionary<string, double> weights = null,
+            RankFusionOptions<TNewResult> options = null)
+        {
+            throw new NotImplementedException();
+        }
+
+        /// <inheritdoc />
+        public virtual IAggregateFluent<TNewResult> RankFusion<TNewResult>(PipelineDefinition<TResult, TNewResult>[] pipelines, RankFusionOptions<TNewResult> options = null)
+        {
+            throw new NotImplementedException();
+        }
+
+        /// <inheritdoc />
+        public virtual IAggregateFluent<TNewResult> RankFusion<TNewResult>(
+            (PipelineDefinition<TResult, TNewResult>, double?)[] pipelinesWithWeights,
+            RankFusionOptions<TNewResult> options = null)
+        {
+            throw new NotImplementedException();
+        }
+
         /// <inheritdoc />
         public virtual IAggregateFluent<TNewResult> ReplaceRoot<TNewResult>(AggregateExpressionDefinition<TResult, TNewResult> newRoot)
         {

src/MongoDB.Driver/Core/Misc/Feature.cs

@@ -82,6 +82,7 @@ public class Feature
         private static readonly Feature __loookupDocuments= new Feature("LoookupDocuments", WireVersion.Server60);
         private static readonly Feature __mmapV1StorageEngine = new Feature("MmapV1StorageEngine", WireVersion.Zero, WireVersion.Server42);
         private static readonly Feature __pickAccumulatorsNewIn52 = new Feature("PickAccumulatorsNewIn52", WireVersion.Server52);
+        private static readonly Feature __rankFusionStage = new Feature("RankFusionStage", WireVersion.Server81);
         private static readonly Feature __regexMatch = new Feature("RegexMatch", WireVersion.Server42);
         private static readonly Feature __round = new Feature("Round", WireVersion.Server42);
         private static readonly Feature __scramSha256Authentication = new Feature("ScramSha256Authentication", WireVersion.Server40);
@@ -386,6 +387,11 @@ public class Feature
         /// </summary>
         public static Feature PickAccumulatorsNewIn52 => __pickAccumulatorsNewIn52;
 
+        /// <summary>
+        /// Gets the $rankFusion feature.
+        /// </summary>
+        public static Feature RankFusionStage => __rankFusionStage;
+
         /// <summary>
         /// Gets the regex match feature.
         /// </summary>

src/MongoDB.Driver/IAggregateFluent.cs

@@ -365,6 +365,41 @@ IAggregateFluent<TNewResult> Lookup<TForeignDocument, TAsElement, TAs, TNewResul
         /// </returns>
         IAggregateFluent<TNewResult> Project<TNewResult>(ProjectionDefinition<TResult, TNewResult> projection);
 
+        /// <summary>
+        /// Appends a $rankFusion stage to the pipeline.
+        /// </summary>
+        /// <typeparam name="TNewResult">The type of the new result.</typeparam>
+        /// <param name="pipelines">The map of named pipelines whose results will be combined. The pipelines must operate on the same collection.</param>
+        /// <param name="weights">The map of pipeline names to non-negative numerical weights determining result importance during combination. Default weight is 1 when unspecified.</param>
+        /// <param name="options">The rankFusion options.</param>
+        /// <returns>The fluent aggregate interface.</returns>
+        IAggregateFluent<TNewResult> RankFusion<TNewResult>(
+            Dictionary<string, PipelineDefinition<TResult, TNewResult>> pipelines,
+            Dictionary<string, double> weights = null,
+            RankFusionOptions<TNewResult> options = null);
+
+        /// <summary>
+        /// Appends a $rankFusion stage to the pipeline. Pipelines will be automatically named as "pipeline1", "pipeline2", etc.
+        /// </summary>
+        /// <typeparam name="TNewResult">The type of the new result.</typeparam>
+        /// <param name="pipelines">The collection of pipelines whose results will be combined. The pipelines must operate on the same collection.</param>
+        /// <param name="options">The rankFusion options.</param>
+        /// <returns>The fluent aggregate interface.</returns>
+        IAggregateFluent<TNewResult> RankFusion<TNewResult>(
+            PipelineDefinition<TResult, TNewResult>[] pipelines,
+            RankFusionOptions<TNewResult> options = null);
+
+        /// <summary>
+        /// Appends a $rankFusion stage to the pipeline. Pipelines will be automatically named as "pipeline1", "pipeline2", etc.
+        /// </summary>
+        /// <typeparam name="TNewResult">The type of the new result.</typeparam>
+        /// <param name="pipelinesWithWeights">The collection of tuples containing (pipeline, weight) pairs. The pipelines must operate on the same collection.</param>
+        /// <param name="options">The rankFusion options.</param>
+        /// <returns>The fluent aggregate interface.</returns>
+        IAggregateFluent<TNewResult> RankFusion<TNewResult>(
+            (PipelineDefinition<TResult, TNewResult>, double?)[] pipelinesWithWeights,
+            RankFusionOptions<TNewResult> options = null);
+
         /// <summary>
         /// Appends a $replaceRoot stage to the pipeline.
         /// </summary>

src/MongoDB.Driver/PipelineDefinitionBuilder.cs

@@ -981,6 +981,65 @@ public static PipelineDefinition<TInput, TOutput> Project<TInput, TIntermediate,
             return pipeline.AppendStage(PipelineStageDefinitionBuilder.Project(projection));
         }
 
+        /// <summary>
+        /// Appends a $rankFusion stage to the pipeline.
+        /// </summary>
+        /// <typeparam name="TInput">The type of the documents.</typeparam>
+        /// <typeparam name="TOutput">The type of the output documents.</typeparam>
+        /// <typeparam name="TIntermediate">The type of the intermediate documents.</typeparam>
+        /// <param name="pipeline">The pipeline.</param>
+        /// <param name="pipelines">The map of named pipelines whose results will be combined. The pipelines must operate on the same collection.</param>
+        /// <param name="weights">The map of pipeline names to non-negative numerical weights determining result importance during combination. Default weight is 1 when unspecified.</param>
+        /// <param name="options">The rankFusion options.</param>
+        /// <returns>A new pipeline with an additional stage.</returns>
+        public static PipelineDefinition<TInput, TOutput> RankFusion<TInput, TIntermediate, TOutput>(
+            this PipelineDefinition<TInput, TIntermediate> pipeline,
+            Dictionary<string, PipelineDefinition<TIntermediate, TOutput>> pipelines,
+            Dictionary<string, double> weights = null,
+            RankFusionOptions<TOutput> options = null)
+        {
+            Ensure.IsNotNull(pipeline, nameof(pipeline));
+            return pipeline.AppendStage(PipelineStageDefinitionBuilder.RankFusion(pipelines, weights, options));
+        }
+
+        /// <summary>
+        /// Appends a $rankFusion stage to the pipeline. Pipelines will be automatically named as "pipeline1", "pipeline2", etc.
+        /// </summary>
+        /// <typeparam name="TInput">The type of the documents.</typeparam>
+        /// <typeparam name="TOutput">The type of the output documents.</typeparam>
+        /// <typeparam name="TIntermediate">The type of the intermediate documents.</typeparam>
+        /// <param name="pipeline">The pipeline.</param>
+        /// <param name="pipelines">The collection of pipelines whose results will be combined. The pipelines must operate on the same collection.</param>
+        /// <param name="options">The rankFusion options.</param>
+        /// <returns>A new pipeline with an additional stage.</returns>
+        public static PipelineDefinition<TInput, TOutput> RankFusion<TInput, TIntermediate, TOutput>(
+            this PipelineDefinition<TInput, TIntermediate> pipeline,
+            PipelineDefinition<TIntermediate, TOutput>[] pipelines,
+            RankFusionOptions<TOutput> options = null)
+        {
+            Ensure.IsNotNull(pipeline, nameof(pipeline));
+            return pipeline.AppendStage(PipelineStageDefinitionBuilder.RankFusion(pipelines, options));
+        }
+
+        /// <summary>
+        /// Appends a $rankFusion stage to the pipeline. Pipelines will be automatically named as "pipeline1", "pipeline2", etc.
+        /// </summary>
+        /// <typeparam name="TInput">The type of the documents.</typeparam>
+        /// <typeparam name="TOutput">The type of the output documents.</typeparam>
+        /// <typeparam name="TIntermediate">The type of the intermediate documents.</typeparam>
+        /// <param name="pipeline">The pipeline.</param>
+        /// <param name="pipelinesWithWeights">The collection of tuples containing (pipeline, weight) pairs. The pipelines must operate on the same collection.</param>
+        /// <param name="options">The rankFusion options.</param>
+        /// <returns>A new pipeline with an additional stage.</returns>
+        public static PipelineDefinition<TInput, TOutput> RankFusion<TInput, TIntermediate, TOutput>(
+            this PipelineDefinition<TInput, TIntermediate> pipeline,
+            (PipelineDefinition<TIntermediate, TOutput>, double?)[] pipelinesWithWeights,
+            RankFusionOptions<TOutput> options = null)
+        {
+            Ensure.IsNotNull(pipeline, nameof(pipeline));
+            return pipeline.AppendStage(PipelineStageDefinitionBuilder.RankFusion(pipelinesWithWeights, options));
+        }
+
         /// <summary>
         /// Appends a $replaceRoot stage to the pipeline.
         /// </summary>

src/MongoDB.Driver/PipelineStageDefinitionBuilder.cs

@@ -1431,6 +1431,111 @@ public static PipelineStageDefinition<TInput, SearchMetaResult> SearchMeta<TInpu
             return stage;
         }
 
+        /// <summary>
+        /// Creates a $rankFusion stage.
+        /// </summary>
+        /// <typeparam name="TInput">The type of the input documents.</typeparam>
+        /// <typeparam name="TOutput">The type of the output documents.</typeparam>
+        /// <param name="pipelines">The map of named pipelines whose results will be combined. The pipelines must operate on the same collection.</param>
+        /// <param name="weights">The map of pipeline names to non-negative numerical weights determining result importance during combination. Default weight is 1 when unspecified.</param>
+        /// <param name="options">The rankFusion options.</param>
+        /// <returns>The stage.</returns>
+        public static PipelineStageDefinition<TInput, TOutput> RankFusion<TInput, TOutput>(
+            Dictionary<string, PipelineDefinition<TInput, TOutput>> pipelines,
+            Dictionary<string, double> weights = null,
+            RankFusionOptions<TOutput> options = null)
+        {
+            Ensure.IsNotNull(pipelines, nameof(pipelines));
+            if (pipelines.Any(pipeline => pipeline.Value == null))
+            {
+                throw new ArgumentNullException(nameof(pipelines), "Pipelines cannot contain a null pipeline.");
+            }
+
+            const string operatorName = "$rankFusion";
+            var stage = new DelegatedPipelineStageDefinition<TInput, TOutput>(
+                operatorName,
+                args =>
+                {
+                    ClientSideProjectionHelper.ThrowIfClientSideProjection(args.DocumentSerializer, operatorName);
+                    var renderedPipelines = new BsonDocument();
+                    foreach (var pipeline in pipelines)
+                    {
+                        renderedPipelines.Add(pipeline.Key, new BsonArray(pipeline.Value.Render(args).Documents));
+                    }
+
+                    var document = new BsonDocument
+                    {
+                        { "input", new BsonDocument("pipelines", renderedPipelines) },
+                        {
+                            "combination", () => new BsonDocument
+                            {
+                                { "weights", new BsonDocument(weights.Select(w => new BsonElement(w.Key, w.Value))) }
+                            },
+                            weights != null
+                        },
+                        { "scoreDetails", true, options?.ScoreDetails == true }
+                    };
+
+                    return new RenderedPipelineStageDefinition<TOutput>(
+                        operatorName,
+                        new BsonDocument(operatorName, document),
+                        options?.OutputSerializer ?? args.SerializerRegistry.GetSerializer<TOutput>());
+                });
+
+            return stage;
+        }
+
+        /// <summary>
+        /// Creates a $rankFusion stage. Pipelines will be automatically named as "pipeline1", "pipeline2", etc.
+        /// </summary>
+        /// <typeparam name="TInput">The type of the input documents.</typeparam>
+        /// <typeparam name="TOutput">The type of the output documents.</typeparam>
+        /// <param name="pipelines">The collection of pipelines whose results will be combined. The pipelines must operate on the same collection.</param>
+        /// <param name="options">The rankFusion options.</param>
+        /// <returns>The stage.</returns>
+        public static PipelineStageDefinition<TInput, TOutput> RankFusion<TInput, TOutput>(
+            PipelineDefinition<TInput, TOutput>[] pipelines,
+            RankFusionOptions<TOutput> options = null)
+        {
+            Ensure.IsNotNull(pipelines, nameof(pipelines));
+
+            var pipelinesMap = new Dictionary<string, PipelineDefinition<TInput, TOutput>>();
+            for (var i = 0; i < pipelines.Length; i++)
+            {
+                pipelinesMap[$"pipeline{i + 1}"] = pipelines[i];
+            }
+            return RankFusion(pipelinesMap, null, options);
+        }
+
+        /// <summary>
+        /// Creates a $rankFusion stage. Pipelines will be automatically named as "pipeline1", "pipeline2", etc.
+        /// </summary>
+        /// <typeparam name="TInput">The type of the input documents.</typeparam>
+        /// <typeparam name="TOutput">The type of the output documents.</typeparam>
+        /// <param name="pipelinesWithWeights">The collection of tuples containing (pipeline, weight) pairs. The pipelines must operate on the same collection.</param>
+        /// <param name="options">The rankFusion options.</param>
+        /// <returns>The stage.</returns>
+        public static PipelineStageDefinition<TInput, TOutput> RankFusion<TInput, TOutput>(
+            (PipelineDefinition<TInput, TOutput> Pipeline, double? Weight)[] pipelinesWithWeights,
+            RankFusionOptions<TOutput> options = null)
+        {
+            Ensure.IsNotNull(pipelinesWithWeights, nameof(pipelinesWithWeights));
+
+            var pipelinesMap = new Dictionary<string, PipelineDefinition<TInput, TOutput>>();
+            var weightsMap = new Dictionary<string, double>();
+            for (var i = 0; i < pipelinesWithWeights.Length; i++)
+            {
+                var pipelineName = $"pipeline{i + 1}";
+                pipelinesMap[pipelineName] = pipelinesWithWeights[i].Pipeline;
+
+                if (pipelinesWithWeights[i].Weight.HasValue)
+                {
+                    weightsMap[pipelineName] = pipelinesWithWeights[i].Weight.Value;
+                }
+            }
+            return RankFusion(pipelinesMap, weightsMap, options);
+        }
+
         /// <summary>
         /// Creates a $replaceRoot stage.
         /// </summary>