DATAMONGO-2320 - Fix aggregation field reference for $filter operator.

We now render field and local variable references correctly when using the $filter aggregation operator.
Prior to this commit field references had been rendered with an additional $ prefix.

Original pull request: #776.

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

@@ -496,7 +496,7 @@ public class ArrayOperators {
 			}
 
 			NestedDelegatingExpressionAggregationOperationContext nea = new NestedDelegatingExpressionAggregationOperationContext(
-					context);
+					context, Collections.singleton(as));
 			return ((AggregationExpression) condition).toDocument(nea);
 		}
 

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

@@ -15,6 +15,11 @@
  */
 package org.springframework.data.mongodb.core.aggregation;
 
+import java.util.Collection;
+import java.util.HashSet;
+import java.util.Set;
+import java.util.stream.Collectors;
+
 import org.bson.Document;
 import org.springframework.data.mongodb.core.aggregation.ExposedFields.FieldReference;
 import org.springframework.data.mongodb.core.aggregation.ExposedFields.ExpressionFieldReference;
@@ -31,16 +36,18 @@ import org.springframework.util.Assert;
 class NestedDelegatingExpressionAggregationOperationContext implements AggregationOperationContext {
 
 	private final AggregationOperationContext delegate;
+	private final Set<String> inners;
 
 	/**
 	 * Creates new {@link NestedDelegatingExpressionAggregationOperationContext}.
 	 *
 	 * @param referenceContext must not be {@literal null}.
 	 */
-	public NestedDelegatingExpressionAggregationOperationContext(AggregationOperationContext referenceContext) {
+	NestedDelegatingExpressionAggregationOperationContext(AggregationOperationContext referenceContext, Collection<Field> inners) {
 
 		Assert.notNull(referenceContext, "Reference context must not be null!");
 		this.delegate = referenceContext;
+		this.inners = inners.stream().map(Field::getName).collect(Collectors.toSet());
 	}
 
 	/*
@@ -58,7 +65,22 @@ class NestedDelegatingExpressionAggregationOperationContext implements Aggregati
 	 */
 	@Override
 	public FieldReference getReference(Field field) {
-		return new ExpressionFieldReference(delegate.getReference(field));
+
+		FieldReference reference = delegate.getReference(field);
+		return !isInnerVariableReference(field) ? reference : new ExpressionFieldReference(delegate.getReference(field)) ;
+	}
+
+	private boolean isInnerVariableReference(Field field) {
+
+		if(inners.isEmpty()) {
+			return false;
+		}
+
+		if(inners.contains(field.getName())) {
+			return true;
+		}
+
+		return inners.stream().anyMatch(it -> field.getTarget().contains(".") && field.getTarget().startsWith(it));
 	}
 
 	/*

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

@@ -18,7 +18,9 @@ package org.springframework.data.mongodb.core.aggregation;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.List;
+import java.util.stream.Collectors;
 
 import org.bson.Document;
 import org.springframework.data.mongodb.core.aggregation.VariableOperators.Let.ExpressionVariable;
@@ -185,7 +187,8 @@ public class VariableOperators {
 			map.putAll(context.getMappedObject(input));
 			map.put("as", itemVariableName);
 			map.put("in",
-					functionToApply.toDocument(new NestedDelegatingExpressionAggregationOperationContext(operationContext)));
+					functionToApply.toDocument(new NestedDelegatingExpressionAggregationOperationContext(operationContext,
+							Collections.singleton(Fields.field(itemVariableName)))));
 
 			return new Document("$map", map);
 		}
@@ -322,12 +325,14 @@ public class VariableOperators {
 
 		private Document getMappedVariable(ExpressionVariable var, AggregationOperationContext context) {
 
-			return new Document(var.variableName, var.expression instanceof AggregationExpression
-					? ((AggregationExpression) var.expression).toDocument(context) : var.expression);
+			return new Document(var.variableName,
+					var.expression instanceof AggregationExpression ? ((AggregationExpression) var.expression).toDocument(context)
+							: var.expression);
 		}
 
 		private Object getMappedIn(AggregationOperationContext context) {
-			return expression.toDocument(new NestedDelegatingExpressionAggregationOperationContext(context));
+			return expression.toDocument(new NestedDelegatingExpressionAggregationOperationContext(context,
+					this.vars.stream().map(var -> Fields.field(var.variableName)).collect(Collectors.toList())));
 		}
 
 		/**

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

@@ -123,6 +123,29 @@ public class FilterExpressionUnitTests {
 		assertThat($filter, is(expected));
 	}
 
+	@Test // DATAMONGO-2320
+	public void shouldConstructFilterExpressionCorrectlyWhenConditionContainsFieldReference() {
+
+		Aggregation agg = Aggregation.newAggregation(Aggregation.project().and((ctx) -> new Document()).as("field-1")
+				.and(filter("items").as("item").by(ComparisonOperators.valueOf("item.price").greaterThan("field-1")))
+				.as("items"));
+
+		Document dbo = agg.toDocument("sales", Aggregation.DEFAULT_CONTEXT);
+
+		List<Object> pipeline = DocumentTestUtils.getAsDBList(dbo, "pipeline");
+		Document $project = DocumentTestUtils.getAsDocument((Document) pipeline.get(0), "$project");
+		Document items = DocumentTestUtils.getAsDocument($project, "items");
+		Document $filter = DocumentTestUtils.getAsDocument(items, "$filter");
+
+		Document expected = Document.parse("{" + //
+				"input: \"$items\"," + //
+				"as: \"item\"," + //
+				"cond: { $gt: [ \"$$item.price\", \"$field-1\" ] }" + //
+				"}");
+
+		assertThat($filter).isEqualTo(new Document(expected));
+	}
+
 	static class Sales {
 
 		List<Object> items;