gistfile1.rb

heaps_of_users_wishlists = UsersWishlist.where(user: user)
heaps_of_groups = BookIdentifierGroup.where(id: heaps_of_users_wishlists.pluck(:book_identifier_group_id)
the_book_identifiers_I_wanna = BookIdentifier.where(group_id: heaps_of_groups.pluck(:id))

puts the_book_identifiers_I_wanna

# This shit triggers 3 queries

heaps_of_users_wishlists = UsersWishlist.where(user: user)
heaps_of_groups = BookIdentifierGroup.joins(:users_wishlists).merge(heaps_of_users_wishlists)
the_book_identifiers_I_wanna = BookIdentifier.joins(:group).merge(heaps_of_groups)

# And this shit triggers 1 query