Mapping Default Values

Motivation

You may want a certain value as default for a column or a expression to invoke on update, insert and so on.

Static default

builder.Property(x => x.FullName)
   .HasDefaultValue("John Smith") // all default value should be `John Smith` in database!
builder.Property(x => x.CreatedDate)
    .HasDefaultValue(DateTime.Now); // should be the datetime value when compiled.

Dynamic default

Server side

Once again, entity framework core translates the form, so we'll need a sql expression to map the dynamic default.

builder.Property(x => x.CreatedDate)
    .HasDefaultValueSql("now()"); // evaluates function `now()` on insert

Client side

Default values can also be generated dynamically from client side with a generator class.

using Microsoft.EntityFrameworkCore.ChangeTracking;
using Microsoft.EntityFrameworkCore.ValueGeneration;

public class CreatedDateGenerator : ValueGenerator<DateTime>
{
    public override DateTime Next(EntityEntry entry)
    {
        return DateTime.UtcNow; // what value should generate, can be more complex
    }

    public override bool GeneratesTemporaryValues => false;
}

builder.Property(x => x.CreatedDate)
    .HasDefaultValue<CreatedDateGenerator>();