The `column` method in ActiveRecord::ConnectionAdapters::TableDefinition
supports adding the `index:` option. The documentation says:
> Instantiates a new column for the table. See connection.add_column for
> available options.
>
> Additional options are:
>
> :index - Create an index for the column. Can be either true or an
> options hash.
So basically the `connection.add_column` method silently ignores the
`index:` option, and whenever we intended to create an index this way,
we didn't.
We're creating a new migration where we properly add the indexes that
weren't added when we intended to.
Thanks to the rubocop-rails team, who added this cop in version 2.11.0
and helped us notice this bug.
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