Add migrations_paths option to model generator

[gmcgibbon]

Summary

Followup on #33760. Adds support for custom migrations path on rails generate model. Example:

bin/rails g model Room capacity:integer --migrations-paths=db/kingston_migrate
      invoke  active_record
      create    db/kingston_migrate/20180830151055_create_rooms.rb

r? @eileencodes
/cc @rafaelfranca

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[gmcgibbon]

This also fixes scaffold, I'll add a test for this.

[eileencodes]

Looks good thanks!

[kaspth]

I'm curious why the user facing API is to pass the migration paths and not the simpler db name?

Perhaps I'm extrapolating from the simple multi db database.yml with the primary database and then the animals.

But wouldn't it be easier to pass --db=animals and let Rails infer the migration paths from database.yml?

[gmcgibbon]

@kaspth I like that idea, maybe we should add support for both? The current implementation would still be useful for edge cases, I think.

[eileencodes]

Let's change it to --database and have that lookup the database migrations paths from the configuration - assuming that the configuration is available to look it up from.

[gmcgibbon]

Sounds good, I'll get to work on it!

[spirobel]

I would have liked --migrations_paths more. For example discourse plugins live in their own directories without having their own db. https://github.com/discourse/discourse/tree/master/plugins/poll/db/migrate Is there maybe another way to achieve something similar?