DATAMONGO-861 - Add support for $cond and $ifNull operators in aggregation operations.

We now support $cond and $ifNull operators for projection and grouping operations. ConditionalOperator and IfNullOperators are AggregationExpressions that can be applied to transform or generate values during aggregation.

TypedAggregation<InventoryItem> agg = newAggregation(InventoryItem.class,
  project().and("discount")
    .transform(ConditionalOperator.newBuilder().when(Criteria.where("qty").gte(250))
      .then(30)
      .otherwise(20))
    .and(ifNull("description", "Unspecified")).as("description")
);

corresponds to

{ "$project": { "discount": { "$cond": { "if": { "$gte": [ "$qty", 250 ] },
        "then": 30, "else": 20 } },
    "description": { "$ifNull": [ "$description", "Unspecified"] }
  }
}

Original Pull Request: #385

Author: mp911de

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

@@ -372,6 +372,68 @@ public static LookupOperation lookup(Field from, Field localField, Field foreign
 		return new LookupOperation(from, localField, foreignField, as);
 	}
 
+	/**
+	 * Creates a new {@link IfNullOperator} for the given {@code field} and {@code replacement} value.
+	 *
+	 * @param field must not be {@literal null}.
+	 * @param replacement must not be {@literal null}.
+	 * @return never {@literal null}.
+	 * @since 1.10
+	 */
+	public static IfNullOperator ifNull(String field, Object replacement) {
+		return IfNullOperator.newBuilder().ifNull(field).thenReplaceWith(replacement);
+	}
+
+	/**
+	 * Creates a new {@link IfNullOperator} for the given {@link Field} and {@link Field} to obtain a value from.
+	 *
+	 * @param field must not be {@literal null}.
+	 * @param replacement must not be {@literal null}.
+	 * @return never {@literal null}.
+	 * @since 1.10
+	 */
+	public static IfNullOperator ifNull(Field field, Field replacement) {
+		return IfNullOperator.newBuilder().ifNull(field).thenReplaceWith(replacement);
+	}
+
+	/**
+	 * Creates a new {@link IfNullOperator} for the given {@link Field} and {@code replacement} value.
+	 *
+	 * @param field must not be {@literal null}.
+	 * @param replacement must not be {@literal null}.
+	 * @return never {@literal null}.
+	 * @since 1.10
+	 */
+	public static IfNullOperator ifNull(Field field, Object replacement) {
+		return IfNullOperator.newBuilder().ifNull(field).thenReplaceWith(replacement);
+	}
+
+	/**
+	 * Creates a new {@link ConditionalOperator} for the given {@link Field} that holds a {@literal boolean} value.
+	 *
+	 * @param booleanField must not be {@literal null}.
+	 * @param then must not be {@literal null}.
+	 * @param otherwise must not be {@literal null}.
+	 * @return never {@literal null}.
+	 * @since 1.10
+	 */
+	public static ConditionalOperator conditional(Field booleanField, Object then, Object otherwise) {
+		return ConditionalOperator.newBuilder().when(booleanField).then(then).otherwise(otherwise);
+	}
+
+	/**
+	 * Creates a new {@link ConditionalOperator} for the given {@link Criteria}.
+	 *
+	 * @param criteria must not be {@literal null}.
+	 * @param then must not be {@literal null}.
+	 * @param otherwise must not be {@literal null}.
+	 * @return never {@literal null}.
+	 * @since 1.10
+	 */
+	public static ConditionalOperator conditional(Criteria criteria, Object then, Object otherwise) {
+		return ConditionalOperator.newBuilder().when(criteria).then(then).otherwise(otherwise);
+	}
+
 	/**
 	 * Creates a new {@link Fields} instance for the given field names.
 	 *