Question

I have a sqlite3 db in a rails app with the following schema

ActiveRecord::Schema.define(:version => 20100816231714) do

  create_table "comments", :force => true do |t|
    t.string   "commenter"
    t.text     "body"
    t.integer  "post_id"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

  create_table "posts", :force => true do |t|
    t.string   "name"
    t.string   "title"
    t.text     "content"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

  create_table "tags", :force => true do |t|
    t.string   "name"
    t.integer  "post_id"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

end

I began with a Post :has_many relationship with tags so each tag has a post_id reference.

I now want to change this relationship to a ‘has_and_belongs_to_many’, I know i have to create the joins table etc…. this isn’t a problem and is working

The problem comes in when i try remove the post_id form the tags table. My migration looks like this:

class RemoveFieldsToTags < ActiveRecord::Migration
   def self.up
     remove_column :tags, :post_id
   end

   def self.down
     add_column :tags, :post_id, :references
   end
 end

When I run rake db:migrate and rake db:migrate:up VERSION= Nothing happens when I run rake db:migrate:down VERSION= I get column:

SQLite3::SQLException: duplicate column name: post_id: ALTER TABLE "tags" ADD "post_id" references

Any one know whats going on?

Answer 1

Like avaynshtok mentions above, it sounds like rails thinks your migrations are up to date (as in, they have all been applied), but to you they are not (the post_id column is still on the tags table).

A common ‘workaround’ to deal with this situation without having to wipe your database is commenting out the ‘down’ method of your migration and running

rake db:migrate:redo

Given the ‘down’ is commented out, it won’t try to add the column again, so it will proceed to reapply the ‘up’ method, removing your ‘post_id’ column. You can then remove the comment on the ‘down’ method and it should all be good.

PS. You might want to look into using a ‘has_many :through’ type of relationship instead of ‘has_and_belongs_to_many’ as well.