Accumulators are not Expressions so they shouldn't derive

There may be a place for a common parent of "tree-like" things, but
Expression is not the correct base. This commit keeps the public API of
accumulators the same so consumers don't need to be modified.

1 files changed, 6 insertions, 8 deletions

diff --git a/src/mongo/db/pipeline/accumulator_last.cpp b/src/mongo/db/pipeline/accumulator_last.cpp
index ea65094d04b..c6fbe859719 100644
--- a/src/mongo/db/pipeline/accumulator_last.cpp
+++ b/src/mongo/db/pipeline/accumulator_last.cpp
@@ -21,19 +21,17 @@
 
 namespace mongo {
 
-    Value AccumulatorLast::evaluate(const Document& pDocument) const {
-        verify(vpOperand.size() == 1);
-
+    void AccumulatorLast::processInternal(const Value& input) {
         /* always remember the last value seen */
-        pValue = vpOperand[0]->evaluate(pDocument);
-
-        return pValue;
+        _last = input;
     }
 
-    AccumulatorLast::AccumulatorLast():
-        AccumulatorSingleValue() {
+    Value AccumulatorLast::getValue() const {
+        return _last;
     }
 
+    AccumulatorLast::AccumulatorLast() {}
+
     intrusive_ptr<Accumulator> AccumulatorLast::create(
         const intrusive_ptr<ExpressionContext> &pCtx) {
         intrusive_ptr<AccumulatorLast> pAccumulator(