In addition to the code change that you mentioned we also need to add a new column musicbrainz_row_id to the database table that holds the users.

I like to confirm if I understood it correctly. 

Firstly we have to make changes to user.py so that for a new user, it returns 3 fields: id (a local acousticbrainz row id), musicbrainz_id (the username), musicbrainz_row_id (the value of data['metabrainz_user_id'] in provider.py). 

Also, for old users, we have to copy-paste the tool from ListenBrainz to get their particular musicbrainz_user_id.

Please guide me if I made a mistake.

In the current database we have 2 fields. id is a local identifier for the row used in acousticbrainz. musicbrainz_id is currently a user's musicbrainz login name. In test_user.py, we call user.create() with a musicbrainz_id (a username, e.g. fuzzy_dunlop), and the row id is returned (e.g. 1)

The new user id is already returned by the MusicBrainz API when we do an oauth login. This value will be in the data object at https://github.com/metabrainz/acousticbrainz-server/blob/6d0099184f00843b3f9c13d94e27803e4d5a28dc/webserver/login/provider.py#L33

Like the change in ListenBrainz, we should at the end have 3 fields: id (a local acousticbrainz row id), musicbrainz_id (the username), musicbrainz_row_id (the value of data['metabrainz_user_id'] in provider.py). Also, we should make sure that we have a tool to query musicbrainz and obtain these row ids for all users who have an account in AcousticBrainz. Again, these scripts exist in the ListenBrainz pull request linked in the main ticket. We can just copy them to AcousticBrainz, as they will only be needed temporarily.

Does this mean that we need to update all the entries by replacing the username with a MiD and how to generate the unique id what features must the identifier carry?

Also, there are two ids in "user" table ( "id" and "musicbrainz_id")  according to the test_user.py (unittest) file it is observed that both the fields take the same values(do they actually contain the same values). How to check what constraints and types are stored in each and see the schema of the tables.

It would be helpful if somebody can elaborate on what has to be done, I am new to this organization and so little guidance could help me work on this issue.