(Repurposing my answer to this stackoverflow question as a blog post)

If you are using Rails 4.x on Heroku and you have your schema format set to sql (which you probably should), you will see this error when running heroku run rake db:migrate

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Aborted!
Error dumping database
.../activerecord-4.0.4/lib/active_record/tasks/postgresql_database_tasks.rb:55:in `structure_dump'
.../activerecord-4.0.4/lib/active_record/tasks/database_tasks.rb:143:in `structure_dump'

This is an interesting bug that, as it turns out, can be ignored.

ActiveRecord will only run the task db:structure:dump if you are using ‘sql’ as your active_record.schema_format. And that rake task (db:structure:dump) will fail on Heroku because the executable pg_dump is (unsurprisingly) not in the binary path. Interestingly enough, Heroku states that db:schema:dump is not supported but if you set the schema format to ruby, it works fine. Of course, you would end up with a schema.rb file on one-off dyno that will not exist shortly after the rake task finished running.

In Rails 3, the dump task would only raise an error is the exit code of the command was 1. On unix based systems if a command is not found, the exit code is 127. So even if the pg_dump command fails on Rails 3 (which it does), it won’t raise an error and it won’t halt the rake task. So anyone using a sql schema format with Rails 3 wouldn’t have this issue because it would fail silently. The code was refactored in Rails 4 to properly raise an error if the dump failed. This means that db:migrate will raise an error on Heroku.

However, even though it errors with rake aborted the DDL from the migration is actually executed and committed.

Here are some possible solutions (if the error really bothers you):

• Ignore the error as the migrations are actually running.
• Since you don’t care about the structure dump on production, set the schema_format to ruby. In config/environments/production.rb:

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config.active_record.schema_format = :ruby

• If for some reason you don’t want to change the config file: add a rake task with the following to suppress the error:

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if Rails.env == 'production'
    Rake::Task["db:structure:dump"].clear
end