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var user = _context.Users.Single(u => u.Id == userId);

user.AssignedInfo = _mapper.Map<AssignedInfo>(assignedInfoDTO);

_context.SaveChanges();

In this case with EF, after the query with the method Single, all other properties but AssignedInfo on the user entity will be overwritten? or are skipped and only the AssignedInfo property is updated?

I ask this because there's a big chance some other users may update those other columns of the table from other endpoints in the api, so I don't want to overwrite other properties than AssignedInfo on this particular endpoint.

Does it work that way or it does update the entire row with all the fields obtained in the query? I just need to update that particular property, and that's the only point where that property is being updated.

devLander
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  • A quick look at the generated SQL would have answered your question much quicker. – Gert Arnold Dec 11 '22 at 09:28
  • I try to give you a fishing rod, not a fish. Knowing how to check generated SQL is an absolute must when working with EF. It reveals lots of what's going on under the hood which otherwise leaves you puzzled. – Gert Arnold Dec 11 '22 at 18:06

1 Answers1

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When you use Single method, EF Core starts to track the selected row, which is user with userId that is intended. Then after changes in user properties, when you use SaveChanges(), EF Core intelligently generates a query which only updates the fields that has been changed, which in your case is AssignedInfo. So the generated query will be something like :

UPDATE [Users] SET [AssignedInfo ] = @p0
WHERE [Id] = @p1;

This is a parameterized query (to prevent from SQL injection) so :

@p0 = _mapper.Map<AssignedInfo>(assignedInfoDTO)

and

@p1 = userId

By the way, you can see the EF Core generated queries just by adding

"Microsoft.EntityFrameworkCore.Database.Command": "Information"

to LogLevel property of Logging property of appsettings.Development.json, so in development mode the queries are shown in the command window or vs output window.

mahooresorkh
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  • Great, I can't see the generated queries yet, don't know why, but thanks for your kind answer. – devLander Dec 11 '22 at 15:34
  • Pay attention to [this post](https://stackoverflow.com/questions/45893732/how-do-you-show-underlying-sql-query-in-ef-core). I hope it helps you. – mahooresorkh Dec 11 '22 at 15:44