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Lists the number of duplicate values
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Lists the number of duplicate values
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I have the following a Rails ActiveRecord association where a lesson has_many books, and books belong_to a lesson. I want to run a SQL command that will tell me how many lessons have multiple books that belong to that lesson.
Ex: Lesson 1 has books 1 and 2, and I want to see how many times that happens.
#having will let you do a #group that filters out Lessons that only have one Book.
Book.group(:lesson_id).having("count(lesson_id) > 1").count
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