Include missing in CompositeAggregationSource (#3602)

Closes #3540

Author: russcam

src/Nest/Aggregations/Bucket/Composite/CompositeAggregationSource.cs

@@ -26,6 +26,14 @@ public interface ICompositeAggregationSource
 		[JsonProperty("missing_bucket")]
 		bool? MissingBucket { get; set; }
 
+
+		/// <summary>
+		/// Allows an explicit value to be set when one is missing for the field.
+		/// </summary>
+		[Obsolete("Will be removed in Elasticsearch 7.x. Use MissingBucket")]
+		[JsonProperty("missing")]
+		object Missing { get; set; }
+
 		/// <summary>
 		/// The name of the source
 		/// </summary>
@@ -60,6 +68,10 @@ protected CompositeAggregationSourceBase(string name) =>
 		/// <inheritdoc />
 		public bool? MissingBucket { get; set; }
 
+		/// <inheritdoc />
+		[Obsolete("Will be removed in Elasticsearch 7.x. Use MissingBucket")]
+		public object Missing { get; set; }
+
 		/// <inheritdoc />
 		public SortOrder? Order { get; set; }
 
@@ -114,7 +126,8 @@ protected CompositeAggregationSourceDescriptorBase(string name, string sourceTyp
 
 		Field ICompositeAggregationSource.Field { get; set; }
 		bool? ICompositeAggregationSource.MissingBucket { get; set; }
-
+		[Obsolete("Will be removed in Elasticsearch 7.x. Use MissingBucket")]
+		object ICompositeAggregationSource.Missing { get; set; }
 		string ICompositeAggregationSource.Name { get; set; }
 		SortOrder? ICompositeAggregationSource.Order { get; set; }
 		string ICompositeAggregationSource.SourceType => _sourceType;
@@ -130,6 +143,10 @@ protected CompositeAggregationSourceDescriptorBase(string name, string sourceTyp
 
 		/// <inheritdoc cref="ICompositeAggregationSource.MissingBucket" />
 		public TDescriptor MissingBucket(bool? includeMissing = true) => Assign(a => a.MissingBucket = includeMissing);
+
+		/// <inheritdoc cref="ICompositeAggregationSource.Missing" />
+		[Obsolete("Will be removed in Elasticsearch 7.x. Use MissingBucket")]
+		public TDescriptor Missing(object missingObject) => Assign(a => a.Missing = missingObject);
 	}
 
 	internal class CompositeAggregationSourceConverter : ReserializeJsonConverter<CompositeAggregationSourceBase, ICompositeAggregationSource>

src/Tests/Tests/Aggregations/Bucket/Composite/CompositeAggregationUsageTests.cs

@@ -319,6 +319,130 @@ protected override void ExpectResponse(ISearchResponse<Project> response)
 		}
 	}
 
+	// hide
+#pragma warning disable 618
+	[SkipVersion("<6.4.0", "Missing added to Composite Aggregation Elasticsearch 6.4.0+")]
+	public class CompositeAggregationMissingUsageTests : ProjectsOnlyAggregationUsageTestBase
+	{
+		public CompositeAggregationMissingUsageTests(ReadOnlyCluster i, EndpointUsage usage) : base(i, usage) { }
+
+		protected override object AggregationJson => new
+		{
+			my_buckets = new
+			{
+				composite = new
+				{
+					sources = new object[]
+					{
+						new
+						{
+							branches = new
+							{
+								terms = new
+								{
+									field = "branches.keyword",
+									order = "asc",
+									missing = "missing_branch"
+								}
+							}
+						},
+					}
+				},
+				aggs = new
+				{
+					project_tags = new
+					{
+						nested = new { path = "tags" },
+						aggs = new
+						{
+							tags = new { terms = new { field = "tags.name" } }
+						}
+					}
+				}
+			}
+		};
+
+		protected override Func<AggregationContainerDescriptor<Project>, IAggregationContainer> FluentAggs => a => a
+			.Composite("my_buckets", date => date
+				.Sources(s => s
+					.Terms("branches", t => t
+						.Field(f => f.Branches.Suffix("keyword"))
+
+						.Missing("missing_branch")
+						.Order(SortOrder.Ascending)
+					)
+				)
+				.Aggregations(childAggs => childAggs
+					.Nested("project_tags", n => n
+						.Path(p => p.Tags)
+						.Aggregations(nestedAggs => nestedAggs
+							.Terms("tags", avg => avg.Field(p => p.Tags.First().Name))
+						)
+					)
+				)
+			);
+
+		protected override AggregationDictionary InitializerAggs =>
+			new CompositeAggregation("my_buckets")
+			{
+				Sources = new List<ICompositeAggregationSource>
+				{
+					new TermsCompositeAggregationSource("branches")
+					{
+						Field = Field<Project>(f => f.Branches.Suffix("keyword")),
+						Missing = "missing_branch",
+						Order = SortOrder.Ascending
+					}
+				},
+				Aggregations = new NestedAggregation("project_tags")
+				{
+					Path = Field<Project>(p => p.Tags),
+					Aggregations = new TermsAggregation("tags")
+					{
+						Field = Field<Project>(p => p.Tags.First().Name)
+					}
+				}
+			};
+
+		protected override void ExpectResponse(ISearchResponse<Project> response)
+		{
+			response.ShouldBeValid();
+
+			var composite = response.Aggregations.Composite("my_buckets");
+			composite.Should().NotBeNull();
+			composite.Buckets.Should().NotBeNullOrEmpty();
+			composite.AfterKey.Should().NotBeNull();
+
+			if (TestConfiguration.Instance.InRange(">=6.3.0"))
+				composite.AfterKey.Should().HaveCount(1).And.ContainKeys("branches");
+
+			var i = 0;
+			var seenMissingBranches = false;
+			foreach (var item in composite.Buckets)
+			{
+				var key = item.Key;
+				key.Should().NotBeNull();
+
+				key.TryGetValue("branches", out string branches).Should().BeTrue("expected to find 'branches' in composite bucket");
+				branches.Should().NotBeNullOrEmpty();
+				if (branches == "missing_branch")
+				{
+					seenMissingBranches = true;
+				}
+
+				var nested = item.Nested("project_tags");
+				nested.Should().NotBeNull();
+
+				var nestedTerms = nested.Terms("tags");
+				nestedTerms.Buckets.Count.Should().BeGreaterThan(0);
+				i++;
+			}
+
+			seenMissingBranches.Should().BeTrue();
+		}
+	}
+#pragma warning restore 618
+
 	//hide
 	[SkipVersion("<6.3.0", "Date histogram source only supports format starting from Elasticsearch 6.3.0+")]
 	public class DateFormatCompositeAggregationUsageTests : ProjectsOnlyAggregationUsageTestBase