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| -rw-r--r-- | guides/source/active_record_migrations.md | 25 |
1 files changed, 22 insertions, 3 deletions
diff --git a/guides/source/active_record_migrations.md b/guides/source/active_record_migrations.md index be5af133ff..e2359a35f1 100644 --- a/guides/source/active_record_migrations.md +++ b/guides/source/active_record_migrations.md @@ -927,9 +927,10 @@ your database schema. It tends to be faster and less error prone to create a new instance of your application's database by loading the schema file via `rails db:schema:load` -than it is to replay the entire migration history. Old migrations may fail to -apply correctly if those migrations use changing external dependencies or rely -on application code which evolves separately from your migrations. +than it is to replay the entire migration history. +[Old migrations](#old-migrations) may fail to apply correctly if those +migrations use changing external dependencies or rely on application code which +evolves separately from your migrations. Schema files are also useful if you want a quick look at what attributes an Active Record object has. This information is not in the model's code and is @@ -1046,3 +1047,21 @@ end This is generally a much cleaner way to set up the database of a blank application. + +Old Migrations +-------------- + +The `schema.rb` or `structure.sql` is a snapshot of the current state of your +database and is the authoritative source for rebuilding that database. This +makes it possible to delete old migration files. + +When you delete migration files in the `db/migrate/` directory, any environment +where `db:migrate` was run when those files still existed will hold a reference +to the migration timestamp specific to them inside an internal Rails database +table named `schema_migrations`. This table is used to keep track of whether +migrations have been executed in a specific environment. + +If you run the `db:migrate:status` command, which displays the status +(up or down) of each migration, you should see `********** NO FILE **********` +displayed next to any deleted migration file which was once executed on a +specific environment but can no longer be found in the `db/migrate` directory. |