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Using Named Scopes Across Models with ActiveRecord#Merge Discussion

General • Asked by Chris Oliver

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You have to fix your latest blog Post excerpt on the home page.
Great site btw.

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Doh! I'll get on that. My markdown parser sure didn't like that video.


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I am doing it with subquery
Like

Author.where(id: Book.available.select(:author_id))

I agree that your version is cleaner and easier to remember,
but you might want to add .distinct after joins to avoid duplicated authors.

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Ah yes, great points! I like that as a subquery and definitely need the distinct for my example as well.

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this query will generate a long SQL query. it's not best practice.


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Well, this is useful, but ugly... Thinking about querying, im not supposed to "merge" anything. IMHO

My wish was Author.books.scope(:available), lol

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Well you can do Author.books.available because that's just querying Book.where(author_id: X).available. But obviously this is for the more complex joining tables case, so at least you're working with the models like you normally would. Not the best, but it saves you from duplication.

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The difference is that Author.books.available will return books, but Author.joins(:books).merge(Book.available) will return authors. Using merge is useful when your desired record set has conditions that are dependant on another table. As stated in the video, you're looking for **Authors** with available books, not available **books**.

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Nice guys.


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Minor nitpick: but I really think you mean `ActiveRecord::Relation#merge`, not `ActiveRecord#merge`


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great! is ugly, but useful, thanks!


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Big thanks! Had a default scope on my Model but rails 4.2.5 didn't unscope it when I was doing "each do" so using the .joins(:model).merge(Model.scope) you suggested instead


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Could do something like:

scope :has_available_books, -> { joins(:books).merge(Book.available) }

Then you could just do:

Author.has_available_books


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Konstantin Ivanov

Need to note that merges on the same attribute overrides each other.

class Book < ActiveRecord::Base
...
scope :red, ->{ where(color: "red") }
scope :green, ->{ where(color: "green") }
end

Author.joins(:books).merge(Book.red).merge(Book.green)
^ would apply only green, because overwrite previous merge


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