feat: insert embedded resources via computed columns

[laurenceisla]

WIP

• Clears item 2 of Multiple requests in a transaction #286 (comment)
• Implements what's mentioned in A relational insert #818 (comment) and A relational insert #818 (comment). There's a discrepancy in both examples, mostly on the tables that are returned by the function. Working on building the exact query to be used in inserting the embedding resource.

[steve-chavez]

739af4e here was merged on 7671d63

[laurenceisla]

It seems that #2933 is definitely going to be an issue when Prefer: return=representation is requested. Doing something like modifying a table inside a function and then doing a JOIN to that same table, uses the OLD data:

-- "tags" is modified inside "upsert_posts"
-- but the LEFT JOIN will show the OLD unmodified "tags"
SELECT * FROM upsert_posts(...) p
LEFT JOIN tags t ON p.id = t.pid;

It could be solved by embedding the RETURNING value of the function, but it cannot be further embedded. For example:

Here, the tags(post, ...) function returns a post table type. As mentioned above the embed will not be possible since the insertion of tags are not known:

curl "http://localhost:3000/posts?columns=name,tags(name)?select=*,tags(name) \
  -H "Content-Type: application/json" \
  -H "Prefer: return=representation" \
  -d '{"name": "my post", "tags": [{"name": "a"},{"name": "b"}]}'

[{
  "name": "my post",
  "tags": []
}]

Making tags(posts, ...) return tags and then using that as an embedding by default (without adding it to ?select=) could be a solution (but further embeds like select=tags(*,other_embed(*)) are not possible):

curl "http://localhost:3000/posts?columns=name,tags(name) \
  -H "Content-Type: application/json" \
  -H "Prefer: return=representation" \
  -d '{"name": "my post", "tags": [{"name": "a"},{"name": "b"}]}'

[{
  "name": "my post",
  "tags": [{"name": "a"},{"name": "b"}]
}]