Add ActiveRecord::QueryMethods#in_order_of.

[kddnewton]

Summary

This allows you to specify an explicit order that you'd like records
returned in based on a SQL expression. By default, this will be accomplished
using a case statement, as in:

Post.in_order_of(:id, [3, 5, 1])

will generate the SQL:

SELECT "posts".* FROM "posts" ORDER BY CASE "posts"."id" WHEN 3 THEN 1 WHEN 5 THEN 2 WHEN 1 THEN 3 ELSE 4 END ASC

However, because this functionality is built into MySQL in the form of the
FIELD function, that connection adapter will generate the following SQL
instead:

SELECT "posts".* FROM "posts" ORDER BY FIELD("posts"."id", 1, 5, 3) DESC

This work largely inspired by the new addition to Enumerable here: #41333. cc @dhh

[dhh]

Post.find([3, 5, 1]) already does this 😄. It's that original behavior that inspired in_order_of! See here:

rails/activerecord/lib/active_record/relation/finder_methods.rb

Line 517 in f95c0b7

def find_some_ordered(ids)

[kddnewton]

@dhh yeah but that returns an array of objects. This returns a relation that you can keep chaining, and add plucks onto the end.

[dhh]

Ah I see. Do you have an example where you know all the IDs you want to fetch, but you want to chain it further still? How would this interact with ORDER etc?

[kddnewton]

@dhh I probably wouldn't be ordering by ID. More likely you have something like:

Post.in_order_of(:type, %w[Draft Published Archived]).order(:created_at).pluck(:name)

which generates

SELECT posts.name
FROM posts
ORDER BY
  CASE posts.type WHEN 'Draft' THEN 1 WHEN 'Published' THEN 2 WHEN 'Archived' THEN 3 ELSE 4 END ASC,
  posts.created_at ASC

in MySQL, it's

SELECT posts.name
FROM posts
ORDER BY
  FIELD(posts.type, 'Archived', 'Published', 'Draft') DESC,
  posts.created_at ASC

It's just another order value, so it goes into the same array.

[kaspth]

Rad as hell! Thanks 🙌

[georgeclaghorn]

Could this avoid reversing values with Arel::Nodes::Ascending?

[kddnewton]

I can't 100% remember why I did this, but it has to do with mysql putting nulls first or last, so it needed to be this way. I'll see if I can find out why.

[p8]

@kaspth I think this change was unintended.
I've made a PR to add the dot back (and add some others).
#42967

[intrip]

@p8 Indeed, thanks!