Be careful with the Active Record where method. When you accidentally pass an empty array to the where method using NOT IN, you probably will not get what you expected:

User.where("id NOT IN (?)", [])
=>  SELECT `users`.* FROM `users` WHERE (id NOT IN (NULL))

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Even though you might expect this to return all records, this actually results none.

Never use the expression id NOT IN (?) without taking care of this case! See below some workarounds.

Rails < 4

Rails < 4 does not provide a pretty workaround.

ids = []

if ids.present?
  User.where("id NOT IN (?)", ids)
else
  User.all
end

Rails >= 4

If you use the same expression as above in Rails >= 4, it is still broken. But you can use the .not method to work around this issue.

User.where.not(id: []).to_sql 
=> SELECT "users".* FROM "users" WHERE (1=1)

User.where.not(id: [1]).to_sql
=> SELECT "users".* FROM "users" WHERE ("users"."id" != 1)

puts User.where.not(id: [1, 2]).to_sql
=> SELECT "users".* FROM "users" WHERE "users"."id" NOT IN (1, 2)