How to validate presence on model with specific field

How can I update all fields in table where the开发者_JS百科 record value is null with validate_presence_of without write all fields one by one?

You mean like default values? Easiest way is to set defaults in your database

add_column :posts, :excerpt, :text, :default => "No excerpt available"

validates_presence_of only checks if a value is present or not.

Optionally, you can set a before_validation hook:

before_validation :set_values

def set_values
  attributes.each do |attr|
    self.send("#{attr}=".to_sym, 'Default value') if attr.nil?
  end
end

I don't think you can use validations for that purpose. Validations are only run when an object is somehow saved or updated to the database so if you already have a bunch of records in a table and then write a validation afterwards, it will not affect the existing database records at all. Unless you try to update them again.

However, if you want to update a bunch of records with one database query them you can use a method call update_all. It could look something like this:

Record.update_all "value = 'default_value'", "value IS NULL"