code: func (t *Transaction) Exists(c *Collection, id string, exists *bool) {
  	if c == nil {
  		t.setError(newNilCollectionError("Exists"))
  		return
  	}
  	t.Command("EXISTS", redis.Args{c.ModelKey(id)}, NewScanBoolHandler(exists))
  }
docs: // Exists sets the value of exists to true if a model exists in the given
  // collection with the given id, and sets it to false otherwise. The first error
  // encountered (if any) will be added to the transaction and returned when
  // the transaction is executed.