DATAMONGO-1533 - Add AggregationExpression derived from SpEL AST.

We added an AggregationExpression that renders a MongoDB Aggregation Framework expression from the AST of a SpEL expression. This allows usage with various stages (eg. $project, $group) throughout the aggregation support.

  // { $and: [ { $gt: [ "$qty", 100 ] }, { $lt: [ "$qty", 250 ] } ] }
  expressionOf("qty > 100 && qty < 250);

  // { $cond : { if : { $gte : [ "$a", 42 ]}, then : "answer", else : "no-answer" } }
  expressionOf("cond(a >= 42, 'answer', 'no-answer')");

Original pull request: #428.

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

@@ -0,0 +1,70 @@
+/*
+ * Copyright 2016. 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
+ *
+ *      http://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.core.aggregation;
+
+import com.mongodb.DBObject;
+import org.springframework.util.Assert;
+
+/**
+ * An {@link AggregationExpression} that renders a MongoDB Aggregation Framework expression from the AST of a
+ * <a href="http://docs.spring.io/spring/docs/current/spring-framework-reference/html/expressions.html">SpEL
+ * expression</a>. <br />
+ * <br />
+ * <strong>Samples:</strong> <br />
+ * <code>
+ * <pre>
+ * // { $and: [ { $gt: [ "$qty", 100 ] }, { $lt: [ "$qty", 250 ] } ] }
+ * expressionOf("qty > 100 && qty < 250);
+ *
+ * // { $cond : { if : { $gte : [ "$a", 42 ]}, then : "answer", else : "no-answer" } }
+ * expressionOf("cond(a >= 42, 'answer', 'no-answer')");
+ * </pre>
+ * </code>
+ * 
+ * @author Christoph Strobl
+ * @see SpelExpressionTransformer
+ * @since 1.10
+ */
+public class AggregationSpELExpression implements AggregationExpression {
+
+	private static final SpelExpressionTransformer TRANSFORMER = new SpelExpressionTransformer();
+	private final String rawExpression;
+	private final Object[] parameters;
+
+	private AggregationSpELExpression(String rawExpression, Object[] parameters) {
+
+		this.rawExpression = rawExpression;
+		this.parameters = parameters;
+	}
+
+	/**
+	 * Creates new {@link AggregationSpELExpression} for the given {@literal expressionString} and {@literal parameters}.
+	 *
+	 * @param expression must not be {@literal null}.
+	 * @param parameters can be empty.
+	 * @return
+	 */
+	public static AggregationSpELExpression expressionOf(String expressionString, Object... parameters) {
+
+		Assert.notNull(expressionString, "ExpressionString must not be null!");
+		return new AggregationSpELExpression(expressionString, parameters);
+	}
+
+	@Override
+	public DBObject toDbObject(AggregationOperationContext context) {
+		return (DBObject) TRANSFORMER.transform(rawExpression, context, parameters);
+	}
+}

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

@@ -37,6 +37,7 @@ import org.springframework.data.mongodb.core.query.Criteria;
 import com.mongodb.BasicDBObject;
 import com.mongodb.BasicDBObjectBuilder;
 import com.mongodb.DBObject;
+import org.springframework.data.mongodb.test.util.BasicDbListBuilder;
 
 /**
  * Unit tests for {@link Aggregation}.
@@ -595,6 +596,27 @@ public class AggregationUnitTests {
 		assertThat(project, isBsonObject().containing("count", 1));
 	}
 
+	/**
+	 * @see DATAMONGO-1533
+	 */
+	@Test
+	public void groupOperationShouldAllowUsageOfDerivedSpELAggregationExpression() {
+
+		DBObject agg = newAggregation( //
+				project("a"), //
+				group("a").first(AggregationSpELExpression.expressionOf("cond(a >= 42, 'answer', 'no-answer')")).as("foosum") //
+		).toDbObject("foo", Aggregation.DEFAULT_CONTEXT);
+
+		@SuppressWarnings("unchecked")
+		DBObject secondProjection = ((List<DBObject>) agg.get("pipeline")).get(1);
+		DBObject fields = getAsDBObject(secondProjection, "$group");
+		assertThat(getAsDBObject(fields, "foosum"), isBsonObject().containing("$first"));
+		assertThat(getAsDBObject(fields, "foosum"), isBsonObject().containing("$first.$cond.if",
+				new BasicDBObject("$gte", new BasicDbListBuilder().add("$a").add(42).get())));
+		assertThat(getAsDBObject(fields, "foosum"), isBsonObject().containing("$first.$cond.then", "answer"));
+		assertThat(getAsDBObject(fields, "foosum"), isBsonObject().containing("$first.$cond.else", "no-answer"));
+	}
+
 	private DBObject extractPipelineElement(DBObject agg, int index, String operation) {
 
 		List<DBObject> pipeline = (List<DBObject>) agg.get("pipeline");