Rails is not saving an attribute that is changed [

问题:

This question already has an answer here:

• New data not persisting to Rails array column on Postgres 3 answers

I am appending some text to a notes field on one of my ActiveRecord::Base models but when I save it, it doesn't get updated:

valve.notes                          
#=> "Level: Top"

valve.notes << "\nDirection: North"

valve.notes                          
#=> "Level: Top\nDirection: North"

valve.save                           
#=> true

valve.reload.notes                   
#=> "Level: Top"

回答1:

Concat doesn't tell ActiveRecord that an Attribute has changed.

Figured it out and wanted to share it here for others (and most likely myself!) in the future.

I didn't know this but ActiveRecord cannot determine that an attribute has been changed (i.e. is dirty) when you concatenate it, either with concat() or <<. And because ActiveRecord only saves, or updates, attributes that have changed (i.e. are dirty), it doesn't update that attribute.

It's a sneaky little gotcha if you're not already aware of it because it not only fails silently, it doesn't think it's failed at all (and perhaps it hasn't, if you ask the ActiveRecord authors :).

valve.notes            
#=> "Level: Top"

valve.notes << "\nDirection: North"

valve.changed?         
#=> false

valve.notes_changed?   
#=> false

valve.save
#=> true

valve.reload.notes
#=> "Level: Top"

You can read more about this on Rails' API Docs.

Solution

To get around this you need to do one of two things:

1. Let ActiveRecord know that the notes attribute has changed (i.e. it is now dirty):

   valve.notes << "\nDirection: North"
   
   valve.changed?                      
   #=> false
   
   valve.notes_will_change!
   
   valve.changed?                      
   #=> true
   

2. Don't use concat() or << to append to your attributes:

   valve.notes = "#{valve.notes}\nDirection: North"
   
   valve.changed?                      
   #=> true
   

Hope that helps at least one other soul.

JP