Add support for avro-type union

[opomuc]

Error catched: unrecognized avro-schema type: ["string",{"type":"map","values":"string"},{"type":"array","items":{"type":"map","values":"string"}}]
Error occured at '/usr/share/tarantool/graphql/tarantool_graphql.lua:784'

[C   ] function 'error' at <[C]:-1>
[Lua ] function 'gql_type' at </usr/share/tarantool/graphql/tarantool_graphql.lua:784>
[Lua ] function 'convert_record_fields' at </usr/share/tarantool/graphql/tarantool_graphql.lua:295>
[Lua ] function 'gql_type' at </usr/share/tarantool/graphql/tarantool_graphql.lua:739>
[Lua ] function 'gql_type' at </usr/share/tarantool/graphql/tarantool_graphql.lua:777>
[Lua ] function 'convert_record_fields' at </usr/share/tarantool/graphql/tarantool_graphql.lua:295>
[Lua ] function 'gql_type' at </usr/share/tarantool/graphql/tarantool_graphql.lua:739>
[Lua ] function 'gql_type' at </usr/share/tarantool/graphql/tarantool_graphql.lua:777>
 [Lua ] function 'convert_record_fields' at </usr/share/tarantool/graphql/tarantool_graphql.lua:295>
[Lua ] function 'gql_type' at </usr/share/tarantool/graphql/tarantool_graphql.lua:739>
[Lua ] function 'convert_record_fields' at </usr/share/tarantool/graphql/tarantool_graphql.lua:295>
[Lua ] function 'gql_type' at </usr/share/tarantool/graphql/tarantool_graphql.lua:739>
[Lua ] function 'parse_cfg' at </usr/share/tarantool/graphql/tarantool_graphql.lua:956>
[Lua ] function 'new' at </usr/share/tarantool/graphql/tarantool_graphql.lua:1131>
***
[C   ] at <[C]:-1>
[C   ] function 'pcall' at <[C]:-1>
[Lua ] function 'cmd_function' at </usr/bin/tarantoolctl:518>
[Lua ] function 'process' at </usr/bin/tarantoolctl:924>
[main] at </usr/bin/tarantoolctl:1271>

[Totktonada]

ETA for merging into master: 2018-04-13 23:59.

[SudoBobo]

GraphQL unions and avro-schema unions (Also see JSON Encoding section below) are different.
Instance of GraphQL union (let's imagine FooObject | BarObject union type):

{ "FooObjectField1": value, "FooObjectField2": anotherValue }
or 
{ "BarObjectField1": value, "BarObjectField2": anotherValue }

with following form of query:

{
    search(...) {
        ... on Foo {
            FooObjectField1
            FooObjectField2
        }
        ... on Bar {
            BarObjectField1
            BarObjectField2
        }
}

Instance of avro-schema union:

{ "FooObject": { "FooObjectField1": value, "FooObjectField2": anotherValue } }
or
{ "BarObject": { "BarObjectField1": value, "BarObjectField2": anotherValue } }

After verbal discussion with @Totktonada we came up with 2 solutions. So we want to convert avro-schema union to GraphQL union. We have the following avro-schema union:
{ "name": "union_field" , "type": ["null", "string", "Foo"] }
The first solution (GraphQL union and queries with types conditional fragments):

{
    ...
    union_field (some_arguments) {
        ... on String (String_value_argument) {
            String
        }
        ... on Foo (some_Foo_arguments) {
            FooObjectField1
            FooObjectField2
        }
}

and union_field also is nullable.
The second one (GraphQL object with nullable fields and no types conditional fragments):

{
    ...
    union_field (some_arguments) {
        string
        Foo (some_Foo_arguments) {
            FooObjectField1
            FooObjectField2
        }
}

So union_field is GraphQL object with all fields nullable.

[SudoBobo]

I think that second solution is better because it is closer to avro-schema conception of unions.

[SudoBobo]

@opomuc
Do you need to filter over union field?

query ($union_type: union_type) {
    user_collection(union_type: $union_type) {
        union_type {
        ...
}

variables {
  "union_type": ...
}

[opomuc]

@SudoBobo I think that we discussed it with @Totktonada and @Kasen: decided that we might need it, but it is currently not a priority