Avoid multiple converter calls when parsing string aggregation stages.

Parsing of string based aggregations lead to multiple invocations of potential converters due to missing reuse of bound parameter value as well as attempts to verify out/merge stages within the pipeline that triggered the stage to be converted into the target document.
The changes in this commit, reduce the number down to 2. One for examining potential expression dependencies and one for the actual conversion and parameter binding.

See #4712
Original pull request: #4717

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/repository/query/AbstractMongoQuery.java

@@ -323,7 +323,8 @@ protected List<AggregationOperation> parseAggregationPipeline(String[] sourcePip
 	}
 
 	private AggregationOperation computePipelineStage(String source, ConvertingParameterAccessor accessor) {
-		return ctx -> ctx.getMappedObject(bindParameters(source, accessor), getQueryMethod().getDomainClass());
+		return new StringAggregationOperation(source, getQueryMethod().getDomainClass(),
+				(it) -> bindParameters(it, accessor));
 	}
 
 	protected Document decode(String source, ParameterBindingContext bindingContext) {

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/repository/query/AbstractReactiveMongoQuery.java

@@ -376,10 +376,8 @@ protected Mono<List<AggregationOperation>> parseAggregationPipeline(String[] pip
 	private Mono<AggregationOperation> computePipelineStage(String source, MongoParameterAccessor accessor,
 			ParameterBindingDocumentCodec codec) {
 
-		return expressionEvaluator(source, accessor, codec).map(it -> {
-			return ctx -> ctx.getMappedObject(decode(it.getT1(), source, accessor, it.getT2()),
-					getQueryMethod().getDomainClass());
-		});
+		return expressionEvaluator(source, accessor, codec).map(
+            it -> new StringAggregationOperation(source, AbstractReactiveMongoQuery.this.getQueryMethod().getDomainClass(), bsonString -> AbstractReactiveMongoQuery.this.decode(it.getT1(), bsonString, accessor, it.getT2())));
 	}
 
 	private Mono<Tuple2<SpELExpressionEvaluator, ParameterBindingDocumentCodec>> expressionEvaluator(String source,

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/repository/query/StringAggregationOperation.java

@@ -0,0 +1,58 @@
+/*
+ * Copyright 2024 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.repository.query;
+
+import java.util.function.Function;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+import org.bson.Document;
+import org.springframework.data.mongodb.core.aggregation.AggregationOperation;
+import org.springframework.data.mongodb.core.aggregation.AggregationOperationContext;
+
+/**
+ * @author Christoph Strobl
+ */
+class StringAggregationOperation implements AggregationOperation {
+
+	private static final Pattern OPERATOR_PATTERN = Pattern.compile("\\$\\w+");
+
+	private final String source;
+	private final Class<?> domainType;
+	private final Function<String, Document> bindFunction;
+
+	StringAggregationOperation(String source, Class<?> domainType, Function<String, Document> bindFunction) {
+
+		this.source = source;
+		this.domainType = domainType;
+		this.bindFunction = bindFunction;
+	}
+
+	@Override
+	public Document toDocument(AggregationOperationContext context) {
+		return context.getMappedObject(bindFunction.apply(source), domainType);
+	}
+
+	@Override
+	public String getOperator() {
+
+		Matcher matcher = OPERATOR_PATTERN.matcher(source);
+		if (matcher.find()) {
+			return matcher.group();
+		}
+		return AggregationOperation.super.getOperator();
+	}
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/util/json/ParameterBindingJsonReader.java

@@ -401,8 +401,9 @@ private BindableValue bindableValueFor(JsonToken token) {
 			if (matcher.find()) {
 
 				int index = computeParameterIndex(matcher.group());
-				bindableValue.setValue(getBindableValueForIndex(index));
-				bindableValue.setType(bsonTypeForValue(getBindableValueForIndex(index)));
+				Object bindableValueForIndex = getBindableValueForIndex(index);
+				bindableValue.setValue(bindableValueForIndex);
+				bindableValue.setType(bsonTypeForValue(bindableValueForIndex));
 				return bindableValue;
 			}
 

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/repository/query/StringBasedAggregationOperationUnitTests.java

@@ -0,0 +1,50 @@
+/*
+ * Copyright 2024 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.repository.query;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.assertj.core.api.Assertions;
+import org.bson.Document;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
+
+/**
+ * @author Christoph Strobl
+ */
+public class StringBasedAggregationOperationUnitTests {
+
+    @ParameterizedTest // GH-4712
+    @ValueSource(strings = { "$project", "'$project'", "\"$project\"" })
+    void extractsAggregationOperatorFromAggregationStringWithoutBindingParameters(String operator) {
+
+        StringAggregationOperation agg = new StringAggregationOperation("{ %s : { 'fn' : 1 } }".formatted(operator),
+            Object.class, (it) -> Assertions.fail("o_O Parameter binding"));
+
+        assertThat(agg.getOperator()).isEqualTo("$project");
+    }
+
+    @Test
+        // GH-4712
+    void fallbackToParameterBindingIfAggregationOperatorCannotBeExtractedFromAggregationStringWithoutBindingParameters() {
+
+        StringAggregationOperation agg = new StringAggregationOperation("{ happy-madison : { 'fn' : 1 } }", Object.class,
+            (it) -> new Document("$project", ""));
+
+        assertThat(agg.getOperator()).isEqualTo("$project");
+    }
+}