[GitHub] davisp commented on a change in pull request #816: Fix mango json index selection

[gi...@git.apache.org]

davisp commented on a change in pull request #816: Fix mango json index selection
URL: https://github.com/apache/couchdb/pull/816#discussion_r139482134
 
 

 ##########
 File path: src/mango/src/mango_selector.erl
 ##########
 @@ -566,3 +567,111 @@ match({[{Field, Cond}]}, Value, Cmp) ->
 
 match({[_, _ | _] = _Props} = Sel, _Value, _Cmp) ->
     erlang:error({unnormalized_selector, Sel}).
+
+
+
+% Returns true if Selector requires all  
+% fields in RequiredFields to exist in any matching documents.
+
+% For each condition in the selector, check
+% whether the field is in RequiredFields.
+% If it is, remove it from RequiredFields and continue
+% until we match then all or run out of selector to
+% match against.
+
+% empty selector
+has_required_fields({[]}, _) ->
+    false;
+
+has_required_fields(Selector, RequiredFields) when not is_list(Selector) ->
+    has_required_fields([Selector], RequiredFields);
+
+% "see" through $and operator. We ignore other
+% combination operators because they can't be used to restrict
+% an index.
+has_required_fields([{[{<<"$and">>, Args}]}], RequiredFields) when is_list(Args) ->
+    has_required_fields(Args, RequiredFields);
+
+has_required_fields([{[{Field, Cond}]} | Rest], RequiredFields) ->
+    case Cond of
+        % $exists:false is a special case - this is the only operator
+        % that explicitly does not require a field to exist
+        {[{<<"$exists">>, false}]} ->
+            has_required_fields(Rest, RequiredFields);
+        _ ->
+            has_required_fields(Rest, lists:delete(Field, RequiredFields))
+    end;
+
+% no more required fields
+has_required_fields(_, []) ->
 
 Review comment:
   Its general practice to place your recursion terminating conditions as the first function clauses.
 
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