Efcore: Question: How does HasComputedColumnSql work?

Created on 11 Oct 2016 · 6Comments · Source: dotnet/efcore

Using the example given in http://ef.readthedocs.io/en/latest/modeling/relational/computed-columns.html

I would've expected the projected SQL to be:
SELECT PersonId, FirstName, LastName, LastName + ', ' + FirstName as DisplayName FROM Persons

but the actual projected SQL is:
SELECT PersonId, FirstName, LastName, DisplayName FROM Persons

I hope I am not being dumb here. I'm just completely blank at the moment. Why do we even need to pass a SQL statement argument in the HasComputedColumnSql method if it's not used?

EDIT: Or is it only used for migrations?

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dealdiane

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Most helpful comment

The method HasComputedColumnSql does not work as developers might expected. Official example https://docs.microsoft.com/en-us/ef/core/modeling/relational/computed-columns is wrong.

art13g on 30 May 2018

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All 6 comments

It works the same way as SQL server's computed columns. You specify computed value for a column and that column value gets computed on insert or updated on edit automatically (you can't change it by hand).

So in your case projected sql is correct, because DisplayName is already calculated and exists in the database as a column with value LastName + ', ' + FirstName.

It is used in migration code to create that kind of column, so you wouldn't have to fallback to altering tables with raw sql.

Muchiachio on 12 Oct 2016

In addition to being used in Migrations, calling HasComputedColumnSql tells the Update pipeline not to save any values for that column and to read back any new value for it after updating the row.

bricelam on 12 Oct 2016

@bricelam it doesn't appear to be the case.
I'm using 2.0.0 and EF core tries to update the property I defined with HasComputedColumnSql.
I had to add [DatabaseGenerated(DatabaseGeneratedOption.Computed)] to the property to stop EF from trying to update the property.
Is that a bug? If not, at least the docs should make that clear.