What is causing this ActiveRecord::ReadOnlyRecord error?

What is causing this ActiveRecord::ReadOnlyRecord error?

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ruby-on-rails

Rails :dependent => :destroy VS :dependent => :delete_all

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Rails: Using build with a has_one association in rails

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ruby-on-rails

Rails migration: t.references with alternative name?

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ruby-on-rails

Build vs new in Rails 3

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How to create has_and_belongs_to_many associations in Factory girl

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ruby-on-rails

Rails association with multiple foreign keys

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ruby-on-rails

Rails - Best-Practice: How to create dependent has_one relations

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Rails has_one :through association

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add associations to exisiting models

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ruby-on-rails

Ruby-on-Rails: Multiple has_many :through possible?

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ruby-on-rails

How to use ActiveAdmin on models using has_many through association?

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ruby-on-rails

Rails 4 find or create by method doesn't work

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ruby-on-rails

rails override default getter for a relationship (belongs_to)

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ruby-on-rails

Validation failed Class must exist

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ruby-on-rails