Handling properties with "DatabaseGenerated" in ASP.NET Core WebAPI with Entity Framework Core

Question

I have the following model class:

public class Item
{
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }

    [Required(ErrorMessage = "You must provide a name.")]
    [MaxLength(255)]
    public string Name { get; set; }
}

When doing GET operations I want to return the object with the Id property, so the user can know what ID to query for update or specific GET. However I DON'T want the client to be able to provide a value for the ID property when doing POST requests and want it to be generated by the database as part of the IDENTITY thing.

How would I achieve that?

What's your issue with your code? It does exactly what you want and if clients fill its Id on insert it will throw exception and you can return Bad Request to client — Arman Ebrahimpour
– Arman Ebrahimpour, Commented May 28, 2020 at 20:44
Because you configure your Id as AutoGenerated, your database will throw an exception when you try to insert record with custom Id — Arman Ebrahimpour
– Arman Ebrahimpour, Commented May 28, 2020 at 21:06

Kishan Vaishnav · Accepted Answer · 2020-05-29 06:11:13Z

1

You cannot manually set the value of an Identity (Auto Increment) column.

Here, in the below example PurchaseId is an Identity column.

purchase.PurchaseId = 6;
_dbContext.Purchases.Add(purchase);
_dbContext.SaveChanges();

When saving changes to the database the EF Core throws Microsoft.EntityFrameworkCore.DbUpdateException with Cannot insert explicit value for identity column in table 'Purchases' when IDENTITY_INSERT is set to OFF. error.

So, by default, you cannot assign the value.

But this is not the issue here.

The bigger issue is that you are exposing the Domain classes to your client. This flaw in API design can lead to more than this problem.

You have to create a DTO e.g. CreateItemDTO whose only responsibility is to contain all the methods required to create an Item in the database.

The same way you should not expose Item class in the GET request. This leads to a problem when API related columns that are not for clients gets exposed. Create a GetItemDTO which would only contain information that is important for the GET request.

CreateItemDTO

public class CreateItemDTO
{
    [Required(ErrorMessage = "You must provide a name.")]
    [MaxLength(255)]
    public string Name { get; set; }
}

Read Exposing domain models over API for more information.

answered May 29, 2020 at 6:11

Kishan Vaishnav

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3 Comments

Shrodinger Over a year ago

But shouldn't the DTO also contain an ID property, so the client can know what is the ID of an object they want to update?

Kishan Vaishnav Over a year ago

@Shrodinger That is why this DTO is called CreateItemDTO. The UpdateItemDTO would contain the {ID, Name}.

Kishan Vaishnav Over a year ago

I know at basic level DTO for each endpoint may seem like overkill. But it will always help you when the requirement for each endpoint will change. Which IMO happens after the client starts to be mature.

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