向Rails模型添加动态属性

Question

In earlier versions of Rails it was possible to add dynamic attributes to a model without the need for there to be an SQL column. The following code worked pre 4.0.

drow = dtab1.drows.create()
drow.write_attribute('value1', 'xxx')
drow.write_attribute('value2', 'yyy')
drow.write_attribute('value3', 'zzz')

But now in v5 I get:

ActiveModel::MissingAttributeError: can't write unknown attribute `value1`

Is there any way to do that now?

Other answers have proposed predefined accessors or replacing the dynamic fields by a "user variables" hash, but that won't work for my situation. They need to be truly dynamic, created at runtime and treated as part of the model.

Answer 1

The write_attribute method updates the attribute in the underlying table. Since your new attributes are dynamic and do not match fields from your model table, no wonder it does not work.

To add attribute dynamically you need to declare it first, and then to call setter as for regular attribute. For example:

attr_name = 'value1'

# Declaring attr_accessor: attr_name
drow.instance_eval { class << self; self end }.send(:attr_accessor, attr_name)

drow.send(attr_name + '=', 'xxx')   # setter
drow.send(attr_name)                # getter

The property will be saved to the instance variable @value1 .

The other way to save dynamic property is to alter that variable directly, without declaring attribute accessor:

drow.instance_variable_set("@#{attr_name}".to_sym, 'xxx') # setter
drow.instance_variable_get("@#{attr_name}".to_sym)        # getter