Add primary_key_default option to create table migrations

[paulyoder]

Motivation / Background

This Pull Request has been created because I want to automatically set the default: argument on create table migrations to "uuid_generate_v4()". The primary key type on my tables are :uuid and I want to use the uuid_generate_v4() method from Postgres to generate the id values.

Detail

It adds the primary_key_default option on the migration generator, which allows an initializer to set the value to automatically use on all create table migrations.

Rails.application.config.generators do |g|
  g.orm(:active_record, primary_key_type: :uuid)
  g.orm(:active_record, primary_key_default: "\"uuid_generate_v4()\"")
end

rails generate migration create_books

class CreateBooks < ActiveRecord::Migration[7.0]
  def change
    create_table :books, id: :uuid, default: "uuid_generate_v4()" do |t|

      t.timestamps
    end
  end
end

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.
• CI is passing.

[bdewater]

This could use an update to https://github.com/rails/rails/blob/main/guides/source/active_record_postgresql.md as well.

[paulyoder]

Thanks @bdewater I'll work on updating that documentation as well

[paulyoder]

@bdewater I updated the active record postgres documentation.

[saiqulhaq]

please fix the conflict 😅

[zzak]

This makes sense to me, if we have --primary-key-type to also have --primary-key-default 🤔

[saiqulhaq]

default: syntax in create_table method is it an existing option? or just introduced in this PR?
when I checked in https://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-create_table
there is no default setting for create_table method
if it's just introduced in this PR, I would prefer to use clearer name, maybe default_uuid
what do you think?

[bdewater]

These are options passed through for table_definition.set_primary_key and uses this allowlist:

rails/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

Lines 1508 to 1510 in 05cb63a

	def valid_primary_key_options # :nodoc:
	[:limit, :default, :precision]
	end

which ultimately is just a column definition.