In early 2015, the poll plugin was writing its data to custom fields on
the post containing the poll. It was later changed to have dedicated SQL
tables and the polls were migrated but we forgot to clean the existing
data.
The migration script is not idempotent due to database constrains on the
poll related objects, namely:
polls: index_polls_on_post_id_and_name (post_id,name) UNIQUE
poll_options: index_poll_options_on_poll_id_and_digest (poll_id,digest) UNIQUE
poll_votes: index_poll_votes_on_poll_id_and_poll_option_id_and_user_id (poll_id,poll_option_id,user_id) UNIQUE
This change skips a particular poll migration if it's already found on
the db.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
