how to say 'is not in' in an active record query

Hey I hope you can help me,

@courses = Course.where(:id != current_user.courses)

I want to get the courses, where the current user is not registered to

@c开发者_StackOverflowourses = Course.where(:id => current_user.courses) gives me the courses where the user has registered to. But I want the opposite

Associations are just fine. I can use current_user.courses or current_user.assignments.

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You probably need something like this:

@courses = Course.where("id NOT IN (?)", current_user.courses)

The reason you can't use a pure array or hash condition is because you're negating ("NOT IN") the query. As far as I know, this can't be done without using the string based syntax. If you just wanted to find matching ("IN") items, you could use the hash form:

@courses = Course.where(:id => current_user.courses)

It's the negating ("NOT IN") part that makes it tricky.

More information can be found in the Active Record Query Interface Guide.

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If you wish to avoid using raw strings, you could use ARel

@courses = Course.where(Course.arel_table[:id].not_in(current_users.courses))

Unfortunately, ARel is not documented very well. I use this as a reference.