Rails: I update migration file then run db:migrate, but my schema isn't updating

问题

I'm trying to add an extra field to one of my tables.

I've added the field in the migration file (under db\migrate), then ran 'rake db:migrate' which ran without troubles. My text editor even told me my schema.db file has been updated and needs to refresh.

The schema file does not contain my new field and any attempts to reference the field from my views fail miserably.

How do I do this? It is possible to update a table with an extra field via rails without having to totally drop and recreate the database again?

回答1:

http://guides.rubyonrails.org/migrations.html#changing-existing-migrations

Occasionally you will make a mistake when writing a migration. If you have already run the migration then you cannot just edit the migration and run the migration again: Rails thinks it has already run the migration and so will do nothing when you run rake db:migrate. You must rollback the migration (for example with rake db:rollback), edit your migration and then run rake db:migrate to run the corrected version.

回答2:

You should always create a new migration file when adding/changing something in the database. This is the purpose of migrations. A migration file should have the ability to make the new change and undo the change. This way if something goes wrong or you changed your mind you can easily roll back to a previous migration.

The following link's sections labeled 'Anatomy of a Migration' and 'Writing a Migration' might be of help to you.

http://guides.rubyonrails.org/migrations.html

回答3:

I did the same thing, I wanted to change a field name and instead of this:

class CreateComments < ActiveRecord::Migration
  def change
    create_table :comments do |t|
      t.string :commenter
      t.text :body

      # this line adds an integer column called `article_id`.
      t.references :article, index: true

      t.timestamps
    end
  end
end

I changed

  t.text :body

to

t.text :comment_body

I tried doing rake

db:migrate

nothing happened as in it went to the command prompt again without any output..., i looked at stack overflow and this guided me to do rake

db:migrate:redo

with out put

== 20141129044056 CreateComments: reverting ===================================
-- drop_table(:comments)
   -> 0.0000s
== 20141129044056 CreateComments: reverted (0.0886s) ==========================

== 20141129044056 CreateComments: migrating ===================================
-- create_table(:comments)
   -> 0.0040s
== 20141129044056 CreateComments: migrated (0.0040s) ==========================

and then I loaded my page/controller with commenter_body instead of body and it loaded perfectly.

I think this is also a solution to the same. I dont know if there is any issue in the underneath workings in the model/DB( i am still very new to RoR,my third day actually...)

回答4:

Solved my own Question..

Basically rather than edit the original migration files generated when you run scaffolding you create a new migration file just for what you want to acheive:

http://guides.rubyonrails.org/migrations.html#creating-a-standalone-migration

回答5:

I was able to regenerate my schema with latter migrations by running rake db:schema:dump

回答6:

Don't know if this applies, but it's worth a shot. Straight from "Agile Development with Rails, 3rd edition":

Sometimes this schema_migrations table can cause you problems. For example, if you create the migration source file and run db:migrate before you add any schema-defining statements to the file, the database will think it has been updated, and the schema info table will contain the new version number.
If you then edit that existing migration file and run db:migrate again, Rails won’t know to apply your new changes. In these circumstances, it’s often easiest to drop the database, re-create it, and rerun your migration(s).

回答7:

Once u do rake db:migrate then again u cannot add column to that file.You have to generate a new migration file by using rails g migration add_columnname_to_tablename for adding that particular column and do rake db:migrate.That's it !!!

来源：https://stackoverflow.com/questions/1416600/rails-i-update-migration-file-then-run-dbmigrate-but-my-schema-isnt-updating