Normalize handling of Undefined vs EOO/missing in agg

Related tickets:
SERVER-6571 replace use of Undefined as missing with EOO
SERVER-6471 ignore nullish values in $min and $max accumulators
SERVER-6144 divide by zero makes field disappear (this solution isn't final)

1 files changed, 4 insertions, 6 deletions

diff --git a/src/mongo/db/pipeline/accumulator_min_max.cpp b/src/mongo/db/pipeline/accumulator_min_max.cpp
index bc48b6fb480..86172cb3338 100755
--- a/src/mongo/db/pipeline/accumulator_min_max.cpp
+++ b/src/mongo/db/pipeline/accumulator_min_max.cpp
@@ -25,17 +25,15 @@ namespace mongo {
         verify(vpOperand.size() == 1);
         Value prhs(vpOperand[0]->evaluate(pDocument));
 
-        /* if this is the first value, just use it */
-        if (pValue.missing())
-            pValue = prhs;
-        else {
+        // nullish values should have no impact on result
+        if (!prhs.nullish()) {
             /* compare with the current value; swap if appropriate */
             int cmp = Value::compare(pValue, prhs) * sense;
-            if (cmp > 0)
+            if (cmp > 0 || pValue.missing()) // missing is lower than all other values
                 pValue = prhs;
         }
 
-        return pValue;
+        return Value();
     }
 
     AccumulatorMinMax::AccumulatorMinMax(int theSense):