Migration replay - #1013
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SimonCropp wants to merge 2 commits into
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Migrations are normally only ever tested against a database built by migrating an empty one. Deployed databases are not like that: a deployment applies state after migrating, such as change tracking, views, or grants, and that state can make DDL that is valid against an empty database fail against a real one. SQL Server refuses to drop a primary key while change tracking is on, so a migration doing that passes every test and then fails on deploy. ReplayRecentMigrations applies the last N migrations one at a time, running a callback after each, so each migration meets the conditions a deployment gives it. Everything before the window is applied in one hop. The one at a time part is load bearing. Applying the window in a single hop and running the callback once at the end is not equivalent, because a table created inside the window never has the state applied before a later migration alters it. ChangeTrackingReplayTests covers exactly that shape, and fails if the implementation is changed to the single hop version. Throws for a database that already has migrations applied, rather than letting EF revert them by running their Down.
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