Allow to use $or with a mango JSON index

Since apache#816, mango JSON index on compound fields can be selected only if
the selector make sure that all the fields listed in the index are
always present. This commit add a special case where all clauses of an
`$or` can ensure that a field is present.

For instance, if I had an index:

[A, B]

is_usable would now return true for the selector:

{
  "A": "foo",
  "$or": {
    "B": "bar",
    "B": "baz"
  }
}

src/mango/src/mango_selector.erl

@@ -596,10 +596,20 @@ has_required_fields(Selector, RequiredFields) when not is_list(Selector) ->
 % We can "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) 
+has_required_fields([{[{<<"$and">>, Args}]}], RequiredFields)
         when is_list(Args) ->
     has_required_fields(Args, RequiredFields);
 
+% We can "see" through $or operator if all the clauses require
+% the same field.
+has_required_fields([{[{<<"$or">>, Args}]} | Rest], [Field | RequiredFields])
+        when is_list(Args) ->
+    Pred = fun(A) -> has_required_fields(A, [Field]) end,
+    case lists:all(Pred, Args) of
+        true -> has_required_fields(Rest, RequiredFields);
+        _ ->  has_required_fields(Rest, [Field | RequiredFields])
+    end;
+
 has_required_fields([{[{Field, Cond}]} | Rest], RequiredFields) ->
     case Cond of
         % $exists:false is a special case - this is the only operator
@@ -662,7 +672,7 @@ has_required_fields_and_false_test() ->
     Normalized = normalize(Selector),
     ?assertEqual(false, has_required_fields(Normalized, RequiredFields)).
 
-has_required_fields_or_test() ->
+has_required_fields_or_false_test() ->
     RequiredFields = [<<"A">>],
     Selector = {[{<<"$or">>,
           [
@@ -673,4 +683,18 @@ has_required_fields_or_test() ->
     Normalized = normalize(Selector),
     ?assertEqual(false, has_required_fields(Normalized, RequiredFields)).
 
--endif.
+has_required_fields_or_true_test() ->
+    RequiredFields = [<<"A">>, <<"B">>, <<"C">>],
+    Selector = {[{<<"A">>, "foo"},
+          {<<"$or">>,
+              [
+                  {[{<<"B">>, <<"bar">>}]},
+                  {[{<<"B">>, <<"baz">>}]}
+              ]
+          },
+		  {<<"C">>, "qux"}
+	]},
+    Normalized = normalize(Selector),
+    ?assertEqual(true, has_required_fields(Normalized, RequiredFields)).
+
+-endif.