[release/5.0] SqlServer Migrations: Don't omit ALTER COLUMN when old type is unknown #23084
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The fix for dotnet#21364 broke migrations created prior to version 3.1.0 which did not specify the old column type. In this case, we don't know whether the column was previously part of an index or not, so we should always include the ALTER COLUMN statement just in case. Note, the old column type is always specified for migrations created with versions 3.1.0 and newer. Fixes dotnet#23045
ajcvickers
approved these changes
Oct 24, 2020
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Fixes #23045
Description
In EF Core 5.0, we fixed issue #21364 which sometimes omitted the ALTER COLUMN statement when it was not needed. Unfortunately, migrations generated using versions of EF Core prior to version 3.1.0 don't contain enough information to always know whether or not the column type was actually changed. This results in the ALTER COLUMN statement being erroneously omitted.
Customer Impact
Anyone with migrations generated prior to EF Core 3.1 who try to apply them using EF Core 5.0 may run into this issue. Their column type will not be changed as expected which can lead to subsequent issues (as reported in the original issue).
How found
Customer reported
Test coverage
We have tests that cover migrations generated using EF Core 3.1 but they didn't cover this specific scenario. This PR adds additional coverage for it.
Regression?
Yes.
Risk
Low. This merely reverts to the behavior of previous versions (i.e. includes the ALTER TABLE statement) when the old column type is null/unknown.