code: func ForeignKeyConstraint(t *testing.T, repo grimoire.Repo) {
  	extra := Extra{}
  	repo.From(extras).MustSave(&extra)
  
  	t.Run("ForeignKeyConstraint", func(t *testing.T) {
  		// inserting
  		err := repo.From(extras).Set("user_id", 1000).Insert(nil)
  		assertConstraint(t, err, errors.ForeignKeyConstraint, "user_id")
  
  		// updating
  		err = repo.From(extras).Find(extra.ID).Set("user_id", 1000).Update(nil)
  		assertConstraint(t, err, errors.ForeignKeyConstraint, "user_id")
  	})
  }
docs: // ForeignKeyConstraint tests foreign key constraint specifications.