Add new jsonb container backend

[shioyama]

This is the first new backend to be added to Mobility since all the original ones were introduced in v0.1.x. I was inspired to add it looking at the Elixir hex package named Trans, which stores all translations for a model in a single depth-2 jsonb column (translations). I've seen this also discussed elsewhere, and thought it would be interesting to implement.

This implementation piggy-backs on parts of the existing Jsonb backend, but in other parts is entirely independent. Specs seem to be passing.

Regarding naming, I'm using the term "container" since this is what Trans uses. In the code I've called the backend "JsonbContainer", but this is quite a mouthful so I'm thinking of just renaming it "container", so you'd use it like this:

class Post
  extend Mobility
  translates :title, backend: :container
end

... or of course just set config.default_backend = :container.

Curious if anyone has any thoughts on this (backend itself, implementation, naming, etc.)

[shioyama]

Note to self: still need to implement for Sequel.

[shioyama]

A query like Post.i18n.where(title: "foo", content: "bar") becomes:

SELECT "posts".* FROM "posts"
WHERE ("posts"."translations" -> 'en' @> '{"title":"foo"}'
AND "posts"."translations" -> 'en' @> '{"content":"bar"}')

This could actually be done with one clause:

SELECT "posts".* FROM "posts"
WHERE ("posts"."translations" -> 'en' @> '{"title":"foo","content":"bar"}')

I'm not sure if there is any performance impact of querying on each key/value separately...

[shioyama]

And here's a query with a nil value, Post.i18n.where(title: "foo", content: nil), where we use the existence operator:

SELECT "posts".* FROM "posts"
  WHERE "posts"."translations" -> 'en' @> '{"title":"foo"}'
  AND NOT ("posts"."translations" ? 'en' AND "posts"."translations" -> 'en' ? 'content')

And a not query, negating the previous one: Post.i18n.where.not(title: "foo", content: nil) becomes:

SELECT "posts".* FROM "posts"
  WHERE NOT (1=0)
  AND "posts"."translations" ? 'en'
  AND "posts"."translations" -> 'en' ? 'title' AND NOT ("posts"."translations" -> 'en' @> '{"title":"foo"}')
  AND "posts"."translations" ? 'en' AND "posts"."translations" -> 'en' ? 'content'

We are thus looking for posts that have a title other than "foo", and which have a content which is non-nil.

Like the jsonb backend, we depend here on the fact that when storing data, we strip any values which are not present. This allows us to use the existence operator (?) without having to worry about keys with blank/null values -- we just assume there are none.

We can also store/query on non-string values (again, like the jsonb backend). e.g. Post.i18n.where(title: {"a" => "b"}) would produce:

SELECT "posts".* FROM "posts"
WHERE "posts"."translations" -> 'en' @> '{"title":{"a":"b"}}'

And this will correctly match posts which have a hash value of {"a": "b"}. The value stored in the translations column in this case would look like this:

{"en"=>{"title"=>{"a"=>"b"}}}

[shioyama]

@pwim I'm curious is this is a backend you would consider using. It has a nice balance of ease-of-use (just one column per translated model), performance (jsonb so fast to query) and relatively low complexity (relative to, say, storing translations on a separate table). Implementation is actually quite simple, even querying.

[shioyama]

This is a lot of code just to fix an issue which is arguably a bug in AR 4.2... debating maybe just skipping the dup specs for this backend in that version of Rails.