Question

I have a table called scheduled_sessions and a Boolean column called instructor_performed to check if the instructor did teach the class or not.

So I need to find all the records where the instructor didn’t teach the class. But I can’t do this: ScheduledSession.where(:instructor_performed => false) because if the cell is blank, it won’t return that record. I just need all records that are NOT true.

Answer 1

It sounds like your instructor_performed column can be true, false, or NULL, so you need to query for false or NULL, like this:

ScheduledSession.where(instructor_performed: [false, nil])

You could avoid this complexity if you’d set up your database table to disallow null values in that column. You can specify this constraint when you create the table in a migration:

add_column :scheduled_session, :instructor_performed, :boolean,
  null: false, default: false

or

create_table :scheduled_session do |t|
  t.boolean :instructor_performed, null: false, default: false
  ...
end

Or you can change the constraint for an existing column:

change_column_null :scheduled_session, :instructor_performed, false, false

In all of the above, we’re setting the column to allow only true or false values, and we’re telling it to use a default value of false. (Without setting the default, you can’t add a no-nulls constraint because your existing data violates it.)

I almost always disallow nulls when I’m setting up boolean columns (unless I truly want tri-state attributes), because it lets me do this to find everything that’s not true:

ScheduledSession.where(instructor_performed: false)

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Note that other answers (now deleted) that encouraged use of an SQL fragment like "instructor_performed != true" won’t work because SQL won’t let you use = or != to match a NULL value. Kind of weird, but them’s the rules. Instead SQL makes you do this:

SELECT * from scheduled_sessions WHERE instructor_performed IS NULL
  OR instructor_performed = FALSE;

which the above Rails where query hides from you somewhat, as long as you’re still aware that you’re searching for two values.