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| -rw-r--r-- | guides/source/migrations.md | 21 |
1 files changed, 21 insertions, 0 deletions
diff --git a/guides/source/migrations.md b/guides/source/migrations.md index 67538b85e8..57db14f30c 100644 --- a/guides/source/migrations.md +++ b/guides/source/migrations.md @@ -889,6 +889,27 @@ class AddFuzzToProduct < ActiveRecord::Migration end ``` +There are other ways in which the above example could have gone badly. + +For example, imagine that Alice creates a migration that selectively +updates the +description+ field on certain products. She runs the +migration, commits the code, and then begins working on the next feature, +which is to add a new column +fuzz+ to the products table. + +She creates two migrations for this new feature, one which adds the new +column, and a second which selectively updates the +fuzz+ column based on +other product attributes. + +These migrations run just fine, but when Bob comes back from his vacation +and calls `rake db:migrate` to run all the outstanding migrations, he gets a +subtle bug: The descriptions have defaults, and the +fuzz+ column is present, +but +fuzz+ is nil on all products. + +The solution is again to use +Product.reset_column_information+ before +referencing the Product model in a migration, ensuring the Active Record's +knowledge of the table structure is current before manipulating data in those +records. + Schema Dumping and You ---------------------- |