ruby on rails - Active Record query based on property of has_many relationship? -

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here's common occurrence. have parent category, category. has_many child categories, books. books have property published, , want categories have published books. how do that?

i loop through categories find them, want better way. in this railscast, suggests using following query: category.joins(:products).merge(product.cheap), or subject.joins(:books).merge(book.published) in example. however, don't have scope published in book.

i have method all_published in book returns published books, tried category.joins(:books).merge(book.all_published) contained duplicate categories. what's best general way solve common problem?

the .joins makes duplicates, whereas .includes not.

in case, following should work:

category.includes(:books).merge(book.all_published)

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