I'm learning Sinatra (1.3.2) and chose to use DataMapper (1.2.0) as ORM and an in-memory SQLite (1.3.6) DB to start.
Two models, Books
and Downloads
, are sharing most attributes, so I looked into declaring a model for STI (Single Table Inheritance) in DataMapper. Reading the docs, this seems a piece of cake thanks to Types::Discriminator.
I abstracted all common ones into DownloadableResource
:
class DownloadableResource
include DataMapper::Resource
property :id, Serial
property :created_at, DateTime
property :modified_at, DateTime
property :active, Boolean, default: true
property :position, Integer
property :title, String, required: true
property :url, URI, required: true
property :description, Text, required: true
property :type, Discriminator
end
Following the example, I thought it's just as easy as specifying what needs to be extended:
class Book < DownloadableResource
property :cover_url, URI
property :authors, String, required: true, length: 255
end
and
class Download < DownloadableResource
property :icon_url, URI
end
but this was giving me the following error:
DataObjects::SyntaxError: duplicate column name: id (code: 1, sql state: , query: ALTER TABLE "downloadable_resources" ADD COLUMN "id" INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT, uri: sqlite3::memory:?scheme=sqlite&user=&password=&host=&port=&query=&fragment=&adapter=sqlite3&path=:memory:)
while removing the id generated another (obvious) error:
DataMapper::IncompleteModelError: DownloadableResource must have a key to be valid
I got around this by adding include DataMapper::Resource
to both Book
and Download
, and then Book
needed a key to be valid, now looking like this:
class Book < DownloadableResource
include DataMapper::Resource
property :id, Serial
property :cover_url, URI
property :authors, String, required: true, length: 255
end
Same goes for Download
, but now the issue is:
DataObjects::SyntaxError: duplicate column name: id (code: 1, sql state: , query: ALTER TABLE "books" ADD COLUMN "id" INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT, uri: sqlite3::memory:?scheme=sqlite&user=&password=&host=&port=&query=&fragment=&adapter=sqlite3&path=:memory:)
Starting to feel like I'm going in circles, what's the proper way to implement Single Table Inheritance in DataMapper?
PS: I have looked at
but I still have this problem.
I would recommend this approach:
module DownloadableResource
def self.included base
base.class_eval do
include DataMapper::Resource
property :created_at, DateTime
property :modified_at, DateTime
property :active, base::Boolean, default: true
property :position, Integer
property :title, String, required: true
property :url, base::URI, required: true
property :description, base::Text, required: true
property :type, base::Discriminator
end
end
end
class Book
include DownloadableResource
property :id, Serial
# other properties
end
class Download
include DownloadableResource
property :id, Serial
# other properties
end