Workaround for AssociationTypeMismatch? - ruby-on-rails

Question

I'm in the middle of a refactoring, where I have written new models for new incoming data that will be behind a feature flag, while simultaneously keeping old models active for incoming data for people who don't yet have the feature flag. The old and new models both interact with the same table in the database.
i.e.
class Poem < ApplicationRecord
belongs_to: :author
belongs_to: :anthology, optional: true
end
class Anthology < ApplicationRecord
belongs_to: :author
has_many: :poems
end
class Author < ApplicationRecord
has_many: :anthologies
has_many: :poems
end
class NewPoem < ApplicationRecord
self.table_name = 'poems'
belongs_to: :author, class_name: 'NewAuthor'
belongs_to: :anthology, class_name: 'NewAnthology', optional: true
end
class NewAnthology < ApplicationRecord
self.table_name = 'anthologies'
belongs_to: :author, class_name: 'NewAuthor'
has_many: :poems, class_name: 'NewPoem'
end
class NewAuthor < ApplicationRecord
self.table_name = 'authors'
has_many: :anthologies, class_name: 'NewAnthology'
has_many: :poems, class_name: 'NewPoem'
end
When I have new books being created, I want to assign the new book to the author
anthology = Anthology.find(1)
#poem = NewPoem.new
#poem.author = anthology.author
Which gives me an error
ActiveRecord::AssociationTypeMismatch (NewAuthor(#70008442274840) expected, got Author(#47031421032620)):
Is there any way to get around this? or will I not be able to associate the models to the old models without a data migration?

Answer 1

You would need to change the first line that fetches the Anthology record. Currently, it fetches a record of the Anthology model as anthology = Anthology.find(1). However, this model refers to the old Author model.
In the next line, #poem = NewPoem.new instantiates an object of the NewPoem model, and the next line tries to assign an author to it. However, #poem.author would be an object of the class NewAuthor, but it is being given a value as #poem.author = anthology.author, where anthology.author still refers to the old Author table.
Changing the first line to anthology = NewAnthology.find(1) would fix the issue. It would fetch the same record as Anthology.find(1) since they query on the same table.
Overall code then becomes:
anthology = NewAnthology.find(1)
#poem = NewPoem.new
#poem.author = anthology.author

Answer 2

It looks like the new models are just supersets of the old ones. You could define NewPoem (and the rest of new models) as:
class NewPoem < Poem
validates :new_colums
def new_cool_stuff
end
end
That way you can avoid to redefine associations.

Answer 3

ActiveRecord actually has a built in Single Table Inheritance mechanism that you can use here.
Start by adding a column named type to the table:
class AddTypeToPoems < ActiveRecord::Migration[5.2]
def change
add_column :poems, :type, :string
add_index :poems, :type
end
end
Then set the classes to inherit:
class Poem < ApplicationRecord
end
class NewPoem < Poem
# gets its table name from the parent class
end
ActiveRecord automatically sets the type column and even uses it when retrieving records:
irb(main):001:0> NewPoem.create
(0.3ms) BEGIN
NewPoem Create (1.3ms) INSERT INTO "poems" ("created_at", "updated_at", "type") VALUES ($1, $2, $3) RETURNING "id" [["created_at", "2019-05-08 19:17:45.086947"], ["updated_at", "2019-05-08 19:17:45.086947"], ["type", "NewPoem"]]
(0.7ms) COMMIT
=> #<NewPoem id: 1, title: nil, created_at: "2019-05-08 19:17:45", updated_at: "2019-05-08 19:17:45", type: "NewPoem">
irb(main):002:0> Poem.first
Poem Load (0.7ms) SELECT "poems".* FROM "poems" ORDER BY "poems"."id" ASC LIMIT $1 [["LIMIT", 1]]
=> #<NewPoem id: 1, title: nil, created_at: "2019-05-08 19:17:45", updated_at: "2019-05-08 19:17:45", type: "NewPoem">
irb(main):003:0>
So even if we use Poem.first it instanciates a NewPoem instance from the result. If you use the finders on the subclasses rails will automatically scope it:
irb(main):003:0> NewPoem.all
NewPoem Load (1.8ms) SELECT "poems".* FROM "poems" WHERE "poems"."type" IN ('NewPoem') LIMIT $1 [["LIMIT", 11]]
If you only want the "old" type you need to use:
Poem.where(type: [nil, 'Poem'])