Accept expression as input for filter aggregation operator.

Closes #4394
Original pull request: #4395

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

@@ -78,7 +78,7 @@ public class ArrayOperators {
 
 		private final @Nullable String fieldReference;
 		private final @Nullable AggregationExpression expression;
-		private final @Nullable Collection values;
+		private final @Nullable Collection<?> values;
 
 		/**
 		 * Creates new {@link ArrayOperatorFactory} for given {@literal fieldReference}.
@@ -213,6 +213,10 @@ public class ArrayOperators {
 				return Filter.filter(fieldReference);
 			}
 
+			if (usesExpression()) {
+				return Filter.filter(expression);
+			}
+
 			Assert.state(values != null, "Values must not be null!");
 			return Filter.filter(new ArrayList<>(values));
 		}
@@ -363,7 +367,7 @@ public class ArrayOperators {
 
 			return usesExpression() ? ArrayToObject.arrayValueOfToObject(expression) : ArrayToObject.arrayToObject(values);
 		}
-		
+
 		/**
 		 * Creates new {@link AggregationExpression} that return the first element in the associated array.
 		 * <strong>NOTE:</strong> Requires MongoDB 4.4 or later.
@@ -379,10 +383,10 @@ public class ArrayOperators {
 
 			return usesExpression() ? First.firstOf(expression) : First.first(values);
 		}
-		
+
 		/**
-		 * Creates new {@link AggregationExpression} that return the last element in the given array.
-		 * <strong>NOTE:</strong> Requires MongoDB 4.4 or later.
+		 * Creates new {@link AggregationExpression} that return the last element in the given array. <strong>NOTE:</strong>
+		 * Requires MongoDB 4.4 or later.
 		 *
 		 * @return new instance of {@link Last}.
 		 * @since 3.4
@@ -633,6 +637,19 @@ public class ArrayOperators {
 			return new FilterExpressionBuilder().filter(field);
 		}
 
+		/**
+		 * Set the {@link AggregationExpression} resolving to an arry to apply the {@code $filter} to.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return never {@literal null}.
+		 * @since 4.2
+		 */
+		public static AsBuilder filter(AggregationExpression expression) {
+
+			Assert.notNull(expression, "Field must not be null");
+			return new FilterExpressionBuilder().filter(expression);
+		}
+
 		/**
 		 * Set the {@literal values} to apply the {@code $filter} to.
 		 *
@@ -669,7 +686,16 @@ public class ArrayOperators {
 		}
 
 		private Object getMappedInput(AggregationOperationContext context) {
-			return input instanceof Field ? context.getReference((Field) input).toString() : input;
+
+			if (input instanceof Field) {
+				return context.getReference((Field) input).toString();
+			}
+
+			if (input instanceof AggregationExpression) {
+				return ((AggregationExpression) input).toDocument(context);
+			}
+
+			return input;
 		}
 
 		private Object getMappedCondition(AggregationOperationContext context) {
@@ -703,6 +729,15 @@ public class ArrayOperators {
 			 * @return
 			 */
 			AsBuilder filter(Field field);
+
+			/**
+			 * Set the {@link AggregationExpression} resolving to an array to apply the {@code $filter} to.
+			 *
+			 * @param expression must not be {@literal null}.
+			 * @return
+			 * @since 3.4.13
+			 */
+			AsBuilder filter(AggregationExpression expression);
 		}
 
 		/**
@@ -793,6 +828,14 @@ public class ArrayOperators {
 				return this;
 			}
 
+			@Override
+			public AsBuilder filter(AggregationExpression expression) {
+
+				Assert.notNull(expression, "Expression must not be null");
+				filter.input = expression;
+				return this;
+			}
+
 			/*
 			 * (non-Javadoc)
 			 * @see org.springframework.data.mongodb.core.aggregation.ArrayOperators.Filter.AsBuilder#as(java.lang.String)
@@ -1348,7 +1391,7 @@ public class ArrayOperators {
 							Assert.notNull(expressions, "PropertyExpressions must not be null");
 
 							return new Reduce(Fields.field(fieldReference), initialValue,
-									Arrays.<AggregationExpression>asList(expressions));
+									Arrays.<AggregationExpression> asList(expressions));
 						}
 					};
 				}
@@ -1708,7 +1751,7 @@ public class ArrayOperators {
 	 * @author Christoph Strobl
 	 * @author Shashank Sharma
 	 * @see <a href=
-	 * "https://docs.mongodb.com/manual/reference/operator/aggregation/in/">https://docs.mongodb.com/manual/reference/operator/aggregation/in/</a>
+	 *      "https://docs.mongodb.com/manual/reference/operator/aggregation/in/">https://docs.mongodb.com/manual/reference/operator/aggregation/in/</a>
 	 * @since 2.2
 	 */
 	public static class In extends AbstractAggregationExpression {
@@ -1797,7 +1840,7 @@ public class ArrayOperators {
 	 *
 	 * @author Christoph Strobl
 	 * @see <a href=
-	 * "https://docs.mongodb.com/manual/reference/operator/aggregation/arrayToObject/">https://docs.mongodb.com/manual/reference/operator/aggregation/arrayToObject/</a>
+	 *      "https://docs.mongodb.com/manual/reference/operator/aggregation/arrayToObject/">https://docs.mongodb.com/manual/reference/operator/aggregation/arrayToObject/</a>
 	 * @since 2.1
 	 */
 	public static class ArrayToObject extends AbstractAggregationExpression {
@@ -1845,7 +1888,7 @@ public class ArrayOperators {
 			return "$arrayToObject";
 		}
 	}
-	
+
 	/**
 	 * {@link AggregationExpression} for {@code $first} that returns the first element in an array. <br />
 	 * <strong>NOTE:</strong> Requires MongoDB 4.4 or later.
@@ -1899,7 +1942,7 @@ public class ArrayOperators {
 			return "$first";
 		}
 	}
-	
+
 	/**
 	 * {@link AggregationExpression} for {@code $last} that returns the last element in an array. <br />
 	 * <strong>NOTE:</strong> Requires MongoDB 4.4 or later.

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

@@ -25,12 +25,8 @@ import org.bson.Document;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
-import org.mockito.Mock;
 import org.mockito.junit.jupiter.MockitoExtension;
-
-import org.springframework.data.mongodb.MongoDatabaseFactory;
 import org.springframework.data.mongodb.core.DocumentTestUtils;
-import org.springframework.data.mongodb.core.convert.DefaultDbRefResolver;
 import org.springframework.data.mongodb.core.convert.MappingMongoConverter;
 import org.springframework.data.mongodb.core.convert.NoOpDbRefResolver;
 import org.springframework.data.mongodb.core.convert.QueryMapper;
@@ -121,6 +117,21 @@ class FilterExpressionUnitTests {
 		assertThat($filter).isEqualTo(new Document(expected));
 	}
 
+	@Test // GH-4394
+	void filterShouldAcceptExpression() {
+
+		Document $filter = ArrayOperators.arrayOf(ObjectOperators.valueOf("data.metadata").toArray()).filter().as("item")
+				.by(ComparisonOperators.valueOf("item.price").greaterThan("field-1")).toDocument(Aggregation.DEFAULT_CONTEXT);
+
+		Document expected = Document.parse("{ $filter : {" + //
+				"input: { $objectToArray: \"$data.metadata\" }," + //
+				"as: \"item\"," + //
+				"cond: { $gt: [ \"$$item.price\", \"$field-1\" ] }" + //
+				"}}");
+
+		assertThat($filter).isEqualTo(expected);
+	}
+
 	private Document extractFilterOperatorFromDocument(Document source) {
 
 		List<Object> pipeline = DocumentTestUtils.getAsDBList(source, "pipeline");