Rails How do I create a default value for attributes in Rails activerecord s model

Question

I want to create a default value for an attribute by defining it in ActiveRecord  By default everytime the record is created  I want to have a default value for attribute  status  I tried to do this   class Task  lt  ActiveRecord  Base   def status  status      status    P      write attribute  status  status    end end   But upon creation I still retrieve this error from the database   ActiveRecord  StatementInvalid  Mysql  Error  Column  status  cannot be null   Therefore I presume the value was not applied to the attribute    What would be the elegant way to do this in Rails   Many thanks

User · Answer

When I need default values its usually for new records before the new action s view is rendered  The following method will set the default values for only new records so that they are available when rendering forms  before save and before create are too late and will not work if you want default values to show up in input fields   after initialize do   if self new record        values will be available for new record forms      self status    P      self featured   true   end end

User · Answer

For column types Rails supports out of the box - like the string in this question - the best approach is to set the column default in the database itself as Daniel Kristensen indicates   Rails will introspect on the DB and initialize the object accordingly   Plus  that makes your DB safe from somebody adding a row outside of your Rails app and forgetting to initialize that column   For column types Rails doesn t support out of the box - e g  ENUM columns - Rails won t be able to introspect the column default   For these cases you do not want to use after initialize  it is called every time an object is loaded from the DB as well as every time an object is created using  new   before create  because it occurs after validation   or before save  because it occurs upon update too  which is usually not what you want    Rather  you want to set the attribute in a before validation on  create  like so   before validation  set status because rails cannot  on   create  def set status because rails cannot   self status      P  end

User · Answer

The solution depends on a few things    Is the default value dependent on other information available at creation time  Can you wipe the database with minimal consequences   If you answered the first question yes  then you want to use Jim s solution  If you answered the second question yes  then you want to use Daniel s solution  If you answered no to both questions  you re probably better off adding and running a new migration    class AddDefaultMigration  lt  ActiveRecord  Migration   def self up      change column  tasks   status   string   default   gt  default value   null   gt  false   end end    string can be replaced with any type that ActiveRecord  Migration recognizes   CPU is cheap so the redefinition of Task in Jim s solution isn t going to cause many problems  Especially in a production environment  This migration is proper way of doing it as it is loaded it and called much less often

User · Answer

Just strengthening Jim s answer  Using presence one can do  class Task  lt  ActiveRecord  Base   before save  default values   def default values     self status   status presence     P    end end

User · Answer

You can do it without writing any code at all    You just need to set the default value for the column in the database  You can do this in your migrations  For example   create table  projects do  t    t string  status   null   gt  false   default   gt   P          t timestamps end   Hope that helps

User · Answer

You can set a default option for the column in the migration       add column  status   string   default   gt   P         OR  You can use a callback  before save  class Task  lt  ActiveRecord  Base   before save  default values   def default values     self status      P    note self status    P  if self status nil  might be safer  per  frontendbeauty    end end

User · Answer

I found a better way to do it now   def status  value     self  status     P   end    In Ruby a method call is allowed to have no parentheses  therefore I should name the local variable into something else  otherwise Ruby will recognize it as a method call

User · Answer

Because I encountered this issue just a little while ago  and the options for Rails 3 0 are a bit different  I ll provide another answer to this question   In Rails 3 0 you want to do something like this   class MyModel  lt  ActiveRecord  Base   after initialize  default values    private     def default values       self name      default value      end end

User · Answer

I would consider using the attr defaults found here  Your wildest dreams will come true

User · Answer

As I see it  there are two problems that need addressing when needing a default value      You need the value present when a new object is initialized   Using after initialize is not suitable because  as stated  it will be called during calls to  find which will lead to a performance hit  You need to persist the default value when saved   Here is my solution     the reader providers a default if nil   but this wont work when saved def status   read attribute  status      P  end    so  define a before validation callback before validation  set defaults protected def set defaults     if a non-default status has been assigned  it will remain     if no value has been assigned  the reader will return the default and assign it     this keeps the default logic DRY   status   status end   I d love to know why people think of this approach

[ruby-on-rails] Rails: How do I create a default value for attributes in Rails activerecord's model?

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