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Are there any strategies for migrating from a separate db per tenant to shared db with scoped tenant_id? In this case each tenant would have overlapping primary keys.

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you add the tenantId to each table and you add it to the primary key of the table too. then you just merge.

“Just”. Depends on the number of tenants and how divergent they are.

I would incrementally bring tenants together into a single DB.

There are also some limitations if using something like rails / activerecord which doesn't support composite primary keys natively

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