How to add new attribute to ActiveRecord

Question

After getting all values from model, I want to add another custom attribute to the ActiveRecord class (this attribute is not a column in db) so that I could use it in view, but

Answer 1

Define virtual attributes as instance variables:

attr_accessor :newattr

Answer 2

If you want this only for your views and do not have any other purpose then you need not to add attr_accessor

@test.all.select('tests.*, "added string" as newattr')

here you are adding newattr attribute for query output of ActiveRecord with a value 'added string'

Answer 3

I think you mean to assign @test to the ActiveRecord query, correct? Try:

@test = MyARClass.select("*, NULL as newatt")
@test.each {|t| t[:newatt] = some_value}

Another related solution is to make it a singleton class method, though you'd have to jump though more hoops to make it writeable and I intuitively feel like this probably incurs more overhead

@test = MyARClass.all
@test.each do t
  def t.newatt
    some_value
  end
end

Using the second method, of course you'd access it via @test.first.newatt, rather than @test.first[:newatt]. You could try redefining t.[] and t.[]=, but this is starting to get really messy.

Answer 4

If it temporary, you can try this:

@test.all.map{ |t| t.attributes.merge({ newatt: "added string" }) }

Answer 5

I met the same issue. and successfully bypass using instance_eval

@test.all

@test.each do |elm|
  elm.instance_eval { @newatt = 'added string' }
end

normally it doesn't run into issue, when use attr_accessor. it appears when other DSL override "newattr=" which cause the issue. In my case, it's money-rails "monetize :newatt"

Explicitly use write_attribute doesn't work as it is the reason to raise exception in rails 4.x

Answer 6

try this

class Test < ActiveRecord::Base
  attr_accessor :newattr
end

you can access it like

@test = Test.new
@test.newattr = "value"

As you may notice this a property, not a hash. so it uses . syntax. however, if you need it to behave like an hash you can do this without defining a new attribute

@test.all
@test.each do |elm|
    new_elm = {}
    new_elm[:newatt] = 'added string'
end

Lastly, I am not exactly sure what you are trying to do. if this doesn't make sense to you, kindly rephrase your question so we can understand the problem better.