feat: add spread embeds

[steve-chavez]

Allows unnesting json objects. Closes #1233.

Instead of getting

GET /grandchild_entities?select=name,child_entities(parent_name:name,entities(grandparent_name:name))&limit=3'

[
{"name":"grandchild entity 1","child_entities":{"parent_name":"child entity 1","entities":{"grandparent_name":"entity 1"}}},
{"name":"grandchild entity 2","child_entities":{"parent_name":"child entity 1","entities":{"grandparent_name":"entity 1"}}},
{"name":"grandchild entity 3","child_entities":{"parent_name":"child entity 2","entities":{"grandparent_name":"entity 1"}}}
]

You can get

GET /grandchild_entities?select=name,..child_entities(parent_name:name,..entities(grandparent_name:name))&limit=3

[
{"name":"grandchild entity 1","parent_name":"child entity 1","grandparent_name":"entity 1"},
{"name":"grandchild entity 2","parent_name":"child entity 1","grandparent_name":"entity 1"},
{"name":"grandchild entity 3","parent_name":"child entity 2","grandparent_name":"entity 1"}
]

Only for many-to-one and one-to-one relationships.

Tasks

• Initial implementation and test
• Refactor - DRY
• Validate to-one relationship
• More tests

[wolfgangwalther]

GET /grandchild_entities?select=name,...child_entities(parent_name:name,...entities(grandparent_name:name))&limit=3

Do we need ... or would two dots be enough?

GET /grandchild_entities?select=name,..child_entities(parent_name:name,..entities(grandparent_name:name))&limit=3

[steve-chavez]

I was reusing this syntax https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax

But two dots seems fine.

[steve-chavez]

The spread operator(...) is even used in GraphQL here: https://spec.graphql.org/June2018/#sec-Language.Fragments.

But two dots seems fine.

I think it would be better to keep that syntax since it's already a popular convention, so will go back to 3 dots in another PR.