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Rails migration to add Primary Key to Existing table

I have a table with two columns employee_id and manager_id without the primary key in Mysql. Employee and Employee_Manager have many to many relationships. In order to update the record, I need a primary key on the Employee_Manager table. Hence, in rails how can generate a migration which adds a column to existing table id(primary key) and set row value for all records in the existing table.

In order to update the record on the association, you don't need an id column in the table. you can always run a query like below:

EmployeeManager.where(employee_id: 120, manager_id: 75).first.update_attributes(from_date: Time.now)

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