Fix problem with multihead connections with nulls

Close #200

Author: SudoBobo

graphql/convert_schema/resolve.lua

@@ -190,6 +190,25 @@ function resolve.gen_resolve_function_multihead(collection_name, connection,
     end
 
     return function(parent, _, info)
+        -- If parent object does not have all source_fields (for any of
+        -- variants) non-nill then we do not resolve variant and just return
+        -- box.NULL
+        for _, variant in ipairs(c.variants) do
+            for _, p in ipairs(variant.parts) do
+                local found = false
+                for field_name, value in pairs(parent) do
+                    if field_name == p.source_field and value ~= nil then
+                        found = true
+                        break
+                    end
+                end
+
+                if not found then
+                    return box.NULL, nil
+                end
+            end
+        end
+
         local v, variant_num, box_field_name = resolve_variant(parent)
         local destination_type = union_types[variant_num]
 

test/common/multihead_conn_null.test.lua

@@ -0,0 +1,16 @@
+#!/usr/bin/env tarantool
+
+local fio = require('fio')
+
+-- require in-repo version of graphql/ sources despite current working directory
+package.path = fio.abspath(debug.getinfo(1).source:match("@?(.*/)")
+    :gsub('/./', '/'):gsub('/+$', '')) .. '/../../?.lua' .. ';' .. package.path
+
+local test_utils = require('test.test_utils')
+local testdata = require('test.testdata.multihead_conn_testdata_with_nulls')
+
+box.cfg({})
+
+test_utils.run_testdata(testdata)
+
+os.exit()