Support Expressions in $slice operator.

This commit allows AggregationExpressions to be used for itemCount and offset of the Slice expression.

Original pull request: #4858
Closes #4857

Author: christophstrobl

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

@@ -1032,35 +1032,68 @@ public Slice itemCount(int nrElements) {
 			return new Slice(append(nrElements));
 		}
 
+		/**
+		 * Slice the number of elements.
+		 *
+		 * @param nrElements An {@link AggregationExpression} that evaluates to a numeric value used as item count.
+		 * @return new instance of {@link Slice}.
+		 * @since 4.5
+		 */
+		public Slice itemCount(AggregationExpression nrElements) {
+			return new Slice(append(nrElements));
+		}
+
 		/**
 		 * Slice using offset and count.
 		 *
 		 * @param position the start position
 		 * @return new instance of {@link SliceElementsBuilder} to create {@link Slice}.
 		 */
-		public SliceElementsBuilder offset(final int position) {
-
-			return new SliceElementsBuilder() {
+		public SliceElementsBuilder offset(int position) {
+			return new SliceElementsBuilder(position);
+		}
 
-				@Override
-				public Slice itemCount(int nrElements) {
-					return new Slice(append(position)).itemCount(nrElements);
-				}
-			};
+		/**
+		 * Slice using offset and count.
+		 *
+		 * @param position the start position
+		 * @return new instance of {@link SliceElementsBuilder} to create {@link Slice}.
+		 */
+		public SliceElementsBuilder offset(AggregationExpression position) {
+			return new SliceElementsBuilder(position);
 		}
 
 		/**
 		 * @author Christoph Strobl
 		 */
-		public interface SliceElementsBuilder {
+		public class SliceElementsBuilder {
+
+			private final Object position;
+
+			SliceElementsBuilder(Object position) {
+				this.position = position;
+			}
 
 			/**
 			 * Set the number of elements given {@literal nrElements}.
 			 *
 			 * @param nrElements
-			 * @return
+			 * @return new instance of {@link Slice}.
 			 */
-			Slice itemCount(int nrElements);
+			public Slice itemCount(int nrElements) {
+				return new Slice(append(position)).itemCount(nrElements);
+			}
+
+			/**
+			 * Slice the number of elements.
+			 *
+			 * @param nrElements An {@link AggregationExpression} that evaluates to a numeric value used as item count.
+			 * @return new instance of {@link Slice}.
+			 * @since 4.5
+			 */
+			public Slice itemCount(AggregationExpression nrElements) {
+				return new Slice(append(position)).itemCount(nrElements);
+			}
 		}
 	}
 

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

@@ -29,6 +29,7 @@
 import org.springframework.data.domain.Range;
 import org.springframework.data.domain.Range.Bound;
 import org.springframework.data.mongodb.core.DocumentTestUtils;
+import org.springframework.data.mongodb.core.aggregation.ArithmeticOperators.Subtract;
 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;
@@ -1064,6 +1065,17 @@ void shouldRenderSliceWithPositionAggregationExpression() {
 		assertThat(agg).isEqualTo(Document.parse("{ $project: { threeFavorites: { $slice: [ \"$favorites\", 2, 3 ] } } }"));
 	}
 
+	@Test // DATAMONGO-4857
+	void shouldRenderSliceWithExpressions() {
+
+		Document agg = project().and(ArrayOperators.arrayOf("favorites").slice()
+				.offset(Subtract.valueOf(ArrayOperators.Size.lengthOfArray("myArray")).subtract(1))
+				.itemCount(ArithmeticOperators.rand())).as("threeFavorites").toDocument(Aggregation.DEFAULT_CONTEXT);
+
+		assertThat(agg).isEqualTo(Document.parse(
+				"{ $project: { threeFavorites: { $slice: [ \"$favorites\", { \"$subtract\":  [ {\"$size\": \"$myArray\"}, 1]}, { $rand : {} } ] } } }"));
+	}
+
 	@Test // DATAMONGO-1536
 	void shouldRenderLiteral() {