irb(main):004:0> u = User.find(1)
D, [2019-02-23T13:36:30.220872 #4] DEBUG -- :   User Load (2.1ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 LIMIT $2  [["id", 1], ["LIMIT", 1]]
=> #<User id: 1, name: "Venya", username: "venik", created_at: "2019-02-20 09:14:18", updated_at: "2019-02-20 09:19:04", email: "venya@mail.ru", password_hash: "598b4f2620256a2e05304ff85d23dfc4f778a4ad8d2de0ac76...", password_salt: "823e00de5cb9c6d50d2baaf1d89d7108", avatar_url: "", headers_color: "#48537b">
irb(main):005:0> u
=> #<User id: 1, name: "Venya", username: "venik", created_at: "2019-02-20 09:14:18", updated_at: "2019-02-20 09:19:04", email: "venya@mail.ru", password_hash: "598b4f2620256a2e05304ff85d23dfc4f778a4ad8d2de0ac76...", password_salt: "823e00de5cb9c6d50d2baaf1d89d7108", avatar_url: "", headers_color: "#48537b">
irb(main):006:0> u.destroy
D, [2019-02-23T13:36:37.092641 #4] DEBUG -- :    (1.8ms)  BEGIN
D, [2019-02-23T13:36:37.096122 #4] DEBUG -- :   User Destroy (2.9ms)  DELETE FROM "users" WHERE "users"."id" = $1  [["id", 1]]
D, [2019-02-23T13:36:37.098032 #4] DEBUG -- :    (1.7ms)  ROLLBACK
Traceback (most recent call last):
        1: from (irb):6
ActiveRecord::InvalidForeignKey (PG::ForeignKeyViolation: ERROR:  update or delete on table "users" violates foreign key constraint "fk_rails_047ab75908" on table "questions")
DETAIL:  Key (id)=(1) is still referenced from table "questions".
: DELETE FROM "users" WHERE "users"."id" = $1