Add support for 'let' and 'pipeline' in $lookup

This commit introduces let and pipline to the Lookup aggregation stage.

Closes: #3322
Original Pull Request: #4272

Author: sangyongchoi

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/Aggregation.java

@@ -50,6 +50,7 @@
  * @author Nikolay Bogdanov
  * @author Gustavo de Geus
  * @author Jérôme Guyon
+ * @author Sangyong Choi
  * @since 1.3
  */
 public class Aggregation {
@@ -664,6 +665,18 @@ public static LookupOperation lookup(Field from, Field localField, Field foreign
 		return new LookupOperation(from, localField, foreignField, as);
 	}
 
+	public static LookupOperation lookup(String from, String localField, String foreignField, String as, List<AggregationOperation> aggregationOperations) {
+		return lookup(field(from), field(localField), field(foreignField), field(as), null, new AggregationPipeline(aggregationOperations));
+	}
+
+	public static LookupOperation lookup(String from, String localField, String foreignField, String as, List<LookupOperation.Let.ExpressionVariable> letExpressionVars, List<AggregationOperation> aggregationOperations) {
+		return lookup(field(from), field(localField), field(foreignField), field(as), new LookupOperation.Let(letExpressionVars), new AggregationPipeline(aggregationOperations));
+	}
+
+	public static LookupOperation lookup(Field from, Field localField, Field foreignField, Field as, LookupOperation.Let let, AggregationPipeline pipeline) {
+		return new LookupOperation(from, localField, foreignField, as, let, pipeline);
+	}
+
 	/**
 	 * Creates a new {@link CountOperationBuilder}.
 	 *

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/LookupOperation.java

@@ -15,6 +15,8 @@
  */
 package org.springframework.data.mongodb.core.aggregation;
 
+import java.util.List;
+
 import org.bson.Document;
 import org.springframework.data.mongodb.core.aggregation.ExposedFields.ExposedField;
 import org.springframework.data.mongodb.core.aggregation.FieldsExposingAggregationOperation.InheritsFieldsAggregationOperation;
@@ -28,6 +30,7 @@
  * @author Alessio Fachechi
  * @author Christoph Strobl
  * @author Mark Paluch
+ * @author Sangyong Choi
  * @since 1.9
  * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/">MongoDB Aggregation Framework:
  *      $lookup</a>
@@ -39,6 +42,11 @@ public class LookupOperation implements FieldsExposingAggregationOperation, Inhe
 	private final Field foreignField;
 	private final ExposedField as;
 
+	@Nullable
+	private final Let let;
+	@Nullable
+	private final AggregationPipeline pipeline;
+
 	/**
 	 * Creates a new {@link LookupOperation} for the given {@link Field}s.
 	 *
@@ -48,7 +56,10 @@ public class LookupOperation implements FieldsExposingAggregationOperation, Inhe
 	 * @param as must not be {@literal null}.
 	 */
 	public LookupOperation(Field from, Field localField, Field foreignField, Field as) {
+		this(from, localField, foreignField, as, null, null);
+	}
 
+	public LookupOperation(Field from, Field localField, Field foreignField, Field as, @Nullable Let let, @Nullable AggregationPipeline pipeline) {
 		Assert.notNull(from, "From must not be null");
 		Assert.notNull(localField, "LocalField must not be null");
 		Assert.notNull(foreignField, "ForeignField must not be null");
@@ -58,6 +69,8 @@ public LookupOperation(Field from, Field localField, Field foreignField, Field a
 		this.localField = localField;
 		this.foreignField = foreignField;
 		this.as = new ExposedField(as, true);
+		this.let = let;
+		this.pipeline = pipeline;
 	}
 
 	@Override
@@ -75,6 +88,14 @@ public Document toDocument(AggregationOperationContext context) {
 		lookupObject.append("foreignField", foreignField.getTarget());
 		lookupObject.append("as", as.getTarget());
 
+		if (let != null) {
+			lookupObject.append("let", let.toDocument(context));
+		}
+
+		if (pipeline != null) {
+			lookupObject.append("pipeline", pipeline.toDocuments(context));
+		}
+
 		return new Document(getOperator(), lookupObject);
 	}
 
@@ -184,4 +205,49 @@ public ForeignFieldBuilder localField(String name) {
 			return this;
 		}
 	}
+
+	public static class Let implements AggregationExpression{
+
+		private final List<ExpressionVariable> vars;
+
+		public Let(List<ExpressionVariable> vars) {
+			Assert.notEmpty(vars, "'let' must not be null or empty");
+			this.vars = vars;
+		}
+
+		@Override
+		public Document toDocument(AggregationOperationContext context) {
+			return toLet();
+		}
+
+		private Document toLet() {
+			Document mappedVars = new Document();
+
+			for (ExpressionVariable var : this.vars) {
+				mappedVars.putAll(getMappedVariable(var));
+			}
+
+			return mappedVars;
+		}
+
+		private Document getMappedVariable(ExpressionVariable var) {
+			return new Document(var.variableName, prefixDollarSign(var.expression));
+		}
+
+		private String prefixDollarSign(String expression) {
+			return "$" + expression;
+		}
+
+		public static class ExpressionVariable {
+
+			private final String variableName;
+
+			private final String expression;
+
+			public ExpressionVariable(String variableName, String expression) {
+				this.variableName = variableName;
+				this.expression = expression;
+			}
+		}
+	}
 }

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/AggregationTests.java

@@ -43,7 +43,6 @@
 
 import org.assertj.core.data.Offset;
 import org.bson.Document;
-import org.bson.types.ObjectId;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
@@ -90,6 +89,7 @@
  * @author Maninder Singh
  * @author Sergey Shcherbakov
  * @author Minsu Kim
+ * @author Sangyong Choi
  */
 @ExtendWith(MongoTemplateExtension.class)
 public class AggregationTests {
@@ -499,7 +499,7 @@ void findStatesWithPopulationOver10MillionAggregationExample() {
 		/*
 		 //complex mongodb aggregation framework example from
 		 https://docs.mongodb.org/manual/tutorial/aggregation-examples/#largest-and-smallest-cities-by-state
-
+		
 		 db.zipcodes.aggregate(
 			 	{
 				   $group: {
@@ -1518,6 +1518,48 @@ void shouldLookupPeopleCorectly() {
 		assertThat(firstItem).containsEntry("linkedPerson.[0].firstname", "u1");
 	}
 
+	@Test
+	void shouldLookupPeopleCorrectlyWithPipeline() {
+		createUsersWithReferencedPersons();
+
+		TypedAggregation<User> agg = newAggregation(User.class, //
+				lookup("person", "_id", "firstname", "linkedPerson", List.of(match(where("firstname").is("u1")))), //
+				sort(ASC, "id"));
+
+		AggregationResults<Document> results = mongoTemplate.aggregate(agg, User.class, Document.class);
+
+		List<Document> mappedResults = results.getMappedResults();
+
+		Document firstItem = mappedResults.get(0);
+
+		assertThat(firstItem).containsEntry("_id", "u1");
+		assertThat(firstItem).containsEntry("linkedPerson.[0].firstname", "u1");
+	}
+
+	@Test
+	void shouldLookupPeopleCorrectlyWithPipelineAndLet() {
+		createUsersWithReferencedPersons();
+
+		TypedAggregation<User> agg = newAggregation(User.class, //
+				lookup(
+						"person",
+						"_id",
+						"firstname",
+						"linkedPerson",
+						List.of(new LookupOperation.Let.ExpressionVariable("personFirstname", "firstname")),
+						List.of(match(where("firstname").is("u1")))),
+				sort(ASC, "id"));
+
+		AggregationResults<Document> results = mongoTemplate.aggregate(agg, User.class, Document.class);
+
+		List<Document> mappedResults = results.getMappedResults();
+
+		Document firstItem = mappedResults.get(0);
+
+		assertThat(firstItem).containsEntry("_id", "u1");
+		assertThat(firstItem).containsEntry("linkedPerson.[0].firstname", "u1");
+	}
+
 	@Test // DATAMONGO-1326
 	void shouldGroupByAndLookupPeopleCorectly() {