Allow base for Table schema definition to be injected into #change_table

[adrianna-chang-shopify]

Summary

This PR is a part of the work to allow users to customize migration behaviour by swapping in their own migration execution strategy objects, which can be used to perform schema statement methods. (See #45324 for more context).

#change_table (when not used in bulk alter query mode) operates by building a Table schema definition, and then yielding that object to the #change_table block. Any methods called within the block are sent to the Table object, which then forwards them to @base. For example, #column:

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

Lines 677 to 684 in ab325a9

	def column(column_name, type, index: nil, **options)
	raise_on_if_exist_options(options)
	@base.add_column(name, column_name, type, **options)
	if index
	index_options = index.is_a?(Hash) ? index : {}
	index(column_name, **index_options)
	end
	end

simply sends the #add_column message to @base, which is the connection adapter.

If apps are using a custom migration execution strategy and defining methods such as #add_column, #add_index, etc., they probably don't want #change_table to send commands to the connection adapter. Instead, it makes more sense that these methods would go to their strategy object. As such, I'm proposing that we simply allow base to be passed as an optional argument to #change_table, with the Table definition constructed using the supplied base. A custom strategy could then do something like this:

class CustomStrategy < ActiveRecord::Migration::DefaultStrategy
  def change_table(table_name, **options, &block)
    super(table_name, self, **options, &block)
  end

  def add_column(table_name, column_name, type, **options)
    # Custom add column
  end

  def add_index(table_name, column_name, **options)
    # Custom add index
  end
end

class TestChangeTable < ActiveRecord::Migration[7.1]
  def change
    change_table(:users, bulk: true) do |t|
      t.column(:name, :string) # This will use the strategy's #add_column method
      t.index(:name, unique: true) # This will use the strategy's #add_index method
    end
  end
end