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Code Salley
Code Salley

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Ruby Single Table Inherency (STI)

From a personal experience, In a recent interview for a rails developer position, i was asked to implement something simple but tricky. eg:

"We need an application to create a post, comments, and replies with only two tables, how will you approach it."

  • All i could think of was creating a boolean column in comments table to trigger replies. After the session, the interviewer gave me a straight tip on what he expected and that's single table inheritance(STI).
  • We've 'Posts table' and 'Comments table', after generating our models and migrating, we can create another class 'Replies' to inherit from 'Comments'. eg

Post model

Rails generate model post title:string
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above we generate a model Post with a title column.

Comments model

Rails generate  model comment title:string type:string
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above is a comments model with title and type, you might be wondering why type? well, we need a column to hold replies type. creating a reply will be saved as a type.

let's generate a replies model.

Rails generate model reply --parent=Comment
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Yes, about the flag used --parent a normal model inherits from active record by default but here we want our model to inherit from Comment model, adding the flag generate something like this

class Reply < Comment
end
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we can comments and replies

# comment
comment = Comment.create(title: "hello comment")

reply
# reply = Reply.create(title: "hello title")
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Reply SQL looks like

INSERT INTO "comments" ("type", "title") VALUES ('Reply', 'hello title')
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You should always try to avoid Single Table Inherency(STI), it makes models associations complicated

Discussion (2)

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leouofa profile image
Leonid Medovyy

Both STI and polymorphic associations are challenging to wrap the head around, but under the right circumstances can be massive timesaver.

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codesalley profile image
Code Salley Author

i agree