SERVER-57914 make getField return missing values

(cherry picked from commit d3d1f3bfe78f39df042bed6bb31b80dbe0b479f8)

3 files changed, 16 insertions, 11 deletions

diff --git a/jstests/aggregation/expressions/expression_get_field.js b/jstests/aggregation/expressions/expression_get_field.js
index 922bbad68ad..017ead1cee1 100644
--- a/jstests/aggregation/expressions/expression_get_field.js
+++ b/jstests/aggregation/expressions/expression_get_field.js
@@ -65,7 +65,6 @@ assertGetFieldFailedWithCode({field: "a"}, 3041703);
 // Test that $getField fails with a document with one or more arguments of incorrect type.
 assertGetFieldFailedWithCode({field: true, input: {a: "b"}}, 5654602);
 assertGetFieldFailedWithCode({field: {"a": 1}, input: {"a": 1}}, 5654601);
-assertGetFieldFailedWithCode({field: "a", input: true}, 3041705);
 assertGetFieldFailedWithCode(5, 5654602);
 assertGetFieldFailedWithCode(true, 5654602);
 assertGetFieldFailedWithCode({field: null, input: {"a": 1}}, 5654602);
@@ -120,6 +119,8 @@ assertGetFieldResultsEq({field: "a.b.c", input: {$const: {"a.b.c": 5}}},
                         [{_id: 0, test: 5}, {_id: 1, test: 5}]);
 assertGetFieldResultsEq({field: "a.b.c", input: {a: {b: {c: 5}}}},
                         [{_id: 0}, {_id: 1}]);  // The test field should evaluate to missing.
+assertGetFieldResultsEq({field: "d", input: {$getField: "c"}},
+                        [{_id: 0, "test": "x"}, {_id: 1, "test": "x"}]);
 
 // Test that $getField works with fields that contain '$'.
 assertGetFieldResultsEq({field: "a$b", input: {"a$b": "b"}},
@@ -150,6 +151,10 @@ assertGetFieldResultsEq({field: "a", input: {}},
 assertGetFieldResultsEq({$const: "$v.."}, [{_id: 0, test: null}, {_id: 1, test: null}]);
 assertGetFieldResultsEq({$const: "$u.."},
                         [{_id: 0}, {_id: 1}]);  // The test field should evaluate to missing.
+assertGetFieldResultsEq({field: "doesNotExist2", input: {$getField: "doesNotExist1"}},
+                        [{_id: 0}, {_id: 1}]);
+assertGetFieldResultsEq({field: "x", input: {$getField: "doesNotExist"}}, [{_id: 0}, {_id: 1}]);
+assertGetFieldResultsEq({field: "a", input: true}, [{_id: 0}, {_id: 1}]);
 
 // Test case where $getField stages are nested.
 assertGetFieldResultsEq(
@@ -160,7 +165,7 @@ assertGetFieldResultsEq(
     [{_id: 0}, {_id: 1}]);
 assertGetFieldResultsEq(
     {field: "a", input: {$getField: {field: "b.d", input: {$const: {"b.c": {a: 5}}}}}},
-    [{_id: 0, test: null}, {_id: 1, test: null}]);
+    [{_id: 0}, {_id: 1}]);
 assertGetFieldResultsEq({field: {$const: "$a"}, input: {$getField: {$const: "$x..$y"}}},
                         [{_id: 0, test: 1}, {_id: 1, test: 1}]);
 assertGetFieldResultsEq({field: {$const: "$b..$c"}, input: {$getField: {$const: "$x..$y"}}},
diff --git a/src/mongo/db/pipeline/expression.cpp b/src/mongo/db/pipeline/expression.cpp
index 268e5f052ac..664c3f81425 100644
--- a/src/mongo/db/pipeline/expression.cpp
+++ b/src/mongo/db/pipeline/expression.cpp
@@ -7314,15 +7314,15 @@ Value ExpressionGetField::evaluate(const Document& root, Variables* variables) c
 
     auto inputValue = _input->evaluate(root, variables);
     if (inputValue.nullish()) {
-        return Value(BSONNULL);
+        if (inputValue.missing()) {
+            return Value();
+        } else {
+            return Value(BSONNULL);
+        }
+    } else if (inputValue.getType() != BSONType::Object) {
+        return Value();
     }
 
-    uassert(3041705,
-            str::stream() << kExpressionName
-                          << " requires 'input' to evaluate to type Object, "
-                             "but got "
-                          << typeName(inputValue.getType()),
-            inputValue.getType() == BSONType::Object);
 
     return inputValue.getDocument().getField(fieldValue.getString());
 }
diff --git a/src/mongo/db/pipeline/expression.h b/src/mongo/db/pipeline/expression.h
index 0438e2d0eb1..64b7c053433 100644
--- a/src/mongo/db/pipeline/expression.h
+++ b/src/mongo/db/pipeline/expression.h
@@ -3512,8 +3512,8 @@ public:
      * Constructs a $getField expression where 'field' is an expression resolving to a constant
      * string Value and 'input' is an expression resolving to an object Value (or null).
      *
-     * If 'input' is nullish, $getField evaluates to null. Furthermore, if 'input' does not contain
-     * 'field', then $getField returns missing.
+     * If 'input' is nullish (but not missing), $getField evaluates to null. Furthermore, if 'input'
+     * does not contain 'field', then $getField returns missing.
      */
     ExpressionGetField(ExpressionContext* const expCtx,
                        boost::intrusive_ptr<Expression> field,