DATAMONGO-1843 - Fix parameter shadowing in ArrayOperators reduce.

Original pull request: #526.

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

@@ -229,8 +229,8 @@ public class ArrayOperators {
 
 				@Override
 				public Reduce startingWith(Object initialValue) {
-					return (usesFieldRef() ? Reduce.arrayOf(fieldReference) : Reduce.arrayOf(expression))
-							.withInitialValue(initialValue).reduce(expression);
+					return (usesFieldRef() ? Reduce.arrayOf(fieldReference)
+							: Reduce.arrayOf(ArrayOperatorFactory.this.expression)).withInitialValue(initialValue).reduce(expression);
 				}
 			};
 		}
@@ -1104,12 +1104,10 @@ public class ArrayOperators {
 		/**
 		 * Start creating new {@link Reduce}.
 		 *
-		 * @param expression must not be {@literal null}.
+		 * @param arrayValueExpression must not be {@literal null}.
 		 * @return
 		 */
-		public static InitialValueBuilder arrayOf(final AggregationExpression expression) {
-
-			Assert.notNull(expression, "AggregationExpression must not be null");
+		public static InitialValueBuilder arrayOf(final AggregationExpression arrayValueExpression) {
 
 			return new InitialValueBuilder() {
 
@@ -1124,14 +1122,14 @@ public class ArrayOperators {
 						public Reduce reduce(AggregationExpression expression) {
 
 							Assert.notNull(expression, "AggregationExpression must not be null");
-							return new Reduce(expression, initialValue, Collections.singletonList(expression));
+							return new Reduce(arrayValueExpression, initialValue, Collections.singletonList(expression));
 						}
 
 						@Override
 						public Reduce reduce(PropertyExpression... expressions) {
 
 							Assert.notNull(expressions, "PropertyExpressions must not be null");
-							return new Reduce(expression, initialValue, Arrays.<AggregationExpression> asList(expressions));
+							return new Reduce(arrayValueExpression, initialValue, Arrays.asList(expressions));
 						}
 					};
 				}

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

@@ -32,10 +32,13 @@ import org.hamcrest.Matchers;
 import org.junit.Test;
 import org.springframework.data.domain.Range;
 import org.springframework.data.mongodb.core.DocumentTestUtils;
+import org.springframework.data.mongodb.core.aggregation.ArrayOperators.Reduce;
 import org.springframework.data.mongodb.core.aggregation.ArrayOperators.Reduce.PropertyExpression;
 import org.springframework.data.mongodb.core.aggregation.ArrayOperators.Reduce.Variable;
+import org.springframework.data.mongodb.core.aggregation.ArrayOperators.Slice;
 import org.springframework.data.mongodb.core.aggregation.ConditionalOperators.Switch.CaseOperator;
 import org.springframework.data.mongodb.core.aggregation.ProjectionOperation.ProjectionOperationBuilder;
+import org.springframework.data.mongodb.core.aggregation.StringOperators.Concat;
 import org.springframework.data.mongodb.core.aggregation.VariableOperators.Let.ExpressionVariable;
 
 /**
@@ -1573,6 +1576,27 @@ public class ProjectionOperationUnitTests {
 				"{ $project : { \"results\": { $reduce: { input: \"$probabilityArr\", initialValue:  { \"sum\" : 5 , \"product\" : 2} , in: { \"sum\": { $add : [\"$$value.sum\", \"$$this\"] }, \"product\": { $multiply: [ \"$$value.product\", \"$$this\" ] } } } } } }")));
 	}
 
+	@Test // DATAMONGO-1843
+	public void shouldRenderReduceWithInputAndInExpressionsCorrectly() {
+
+		Document exprected = Document.parse(
+				"{ \"$project\" : { \"results\" : { \"$reduce\" : { \"input\" : { \"$slice\" : [\"$array\", 5] }, \"initialValue\" : \"\", \"in\" : { \"$concat\" : [\"$$value\", \"/\", \"$$this\"] } } } } }");
+
+		Reduce reduceEntryPoint = Reduce.arrayOf(Slice.sliceArrayOf("array").itemCount(5)) //
+				.withInitialValue("") //
+				.reduce(Concat.valueOf("$$value").concat("/").concatValueOf("$$this"));
+
+		Reduce arrayEntryPoint = ArrayOperators.arrayOf(Slice.sliceArrayOf("array").itemCount(5)) //
+				.reduce(Concat.valueOf("$$value").concat("/").concatValueOf("$$this")) //
+				.startingWith("");
+
+		assertThat(project().and(reduceEntryPoint).as("results").toDocument(Aggregation.DEFAULT_CONTEXT),
+				Matchers.is(exprected));
+
+		assertThat(project().and(arrayEntryPoint).as("results").toDocument(Aggregation.DEFAULT_CONTEXT),
+				Matchers.is(exprected));
+	}
+
 	@Test // DATAMONGO-1548
 	public void shouldRenderZipCorrectly() {