code: func UniqueConstraint(ch *Changeset, field string, opts ...Option) {
  	options := Options{
  		message: UniqueConstraintMessage,
  		name:    field,
  		exact:   false,
  	}
  	options.apply(opts)
  
  	ch.constraints = append(ch.constraints, Constraint{
  		Field:   field,
  		Message: strings.Replace(options.message, "{field}", field, 1),
  		Code:    options.code,
  		Name:    options.name,
  		Exact:   options.exact,
  		Kind:    errors.UniqueConstraint,
  	})
  }
docs: // UniqueConstraint adds an unique constraint to changeset.