How to implement Asp.net identity with existing database

Question

I have to implement asp.net identity in my asp.net core project.I have go through maximum related topics by googling my requirement.but there is not most suitable solution I could find. If I create project using individual authentication,then i have to apply database migration to my existing database in sql server,tables are then created,but context class is now application user is also added to my project. and then using database first approach when I create classes of my database using reverse engineering then another mydatabasenamecontext class is added to project.and then all is do mess with these context classes and their configuration. I want that after applying migration,when I use database first approach all existing context class reference must be updated,only one flow of application to db and vice versa can be implement using ?