C#•13mo ago

✅ What's the point of ModelSnapshot in ef core migrations

I'm trying to move my migrations to a separate projects in case I want to change a db provider. I've moved my existing migrations to the separate project and that's what I've got now:

Application.Core <-- ApplicationDbContext
Application.Migrations.MsSql <-- References Application.Persistence, contains migrations

Application.Core <-- ApplicationDbContext
Application.Migrations.MsSql <-- References Application.Persistence, contains migrations

In my Application.Core project I've added the following config for DI:

services.AddDbContext(o => o.UseSqlServer("connectionString"));

services.AddDbContext(o => o.UseSqlServer("connectionString"));

I didn't add x => x.MigrationsAssembly("WebApplication1.Migrations") call as it shown in the docs (https://learn.microsoft.com/en-us/ef/core/managing-schemas/migrations/projects) My questions is: how come EF still works even though it doesn't reference Migrations project where AppContextModelSnapshot is stored? And what are potential issues with my approach?

22 Replies

Omnissiah•13mo ago

i use a different assembly/project for migrations what do you mean by "ef core still works"

cumslvt13OP•13mo ago

I mean that I still can query database altough project where dbcontext is doesn't reference project with migrations, which contains AppContextModelSnapshot which has modelbuilder setup

Omnissiah•13mo ago

migrations and snapshot aren't needed for dbcontext

cumslvt13OP•13mo ago

so the modebuilder configuration is not actually used during runtime? Because this kinda confused me, because I can write the same configuration (for example for replationships) if I override it

Jimmacle•13mo ago

are you migrating your database at runtime? migrations are not your model, they describe how to modify your database when you change your model

JakenVeina•13mo ago

The ModelSnapshot is not used at runtime. It's used by the designer tool to generate migrations, and possibly some of its other functions the ModelBuilder configuration is very much used at runtime, and is also used to generate ModelSnapshots

cumslvt13OP•13mo ago

No, migrations are being run in that separate project with migrations in CI Got it, thanks. Got confused because both BuildModel and OnModelCreating are accepting ModelBuilder

JakenVeina•13mo ago

where's BuildModel?

cumslvt13OP•13mo ago

 [DbContext(typeof(AppDbContext))]
partial class AppContextModelSnapshot : ModelSnapshot
{
    protected override void BuildModel(ModelBuilder modelBuilder)
    {
        // omitted
    }
}

 [DbContext(typeof(AppDbContext))]
partial class AppContextModelSnapshot : ModelSnapshot
{
    protected override void BuildModel(ModelBuilder modelBuilder)
    {
        // omitted
    }
}

JakenVeina•13mo ago

hmm oh so, that method's not empty, right?

cumslvt13OP•13mo ago

yeah, I've omitted it

JakenVeina•13mo ago

yeah, the snapshot is just another method of building a model so, like

cumslvt13OP•13mo ago

I was confused because actually BuidModel can contain the same configurations as OnModelCreating can

JakenVeina•13mo ago

yeah, that's by design

cumslvt13OP•13mo ago

But now I understand that it's purely for migrations

JakenVeina•13mo ago

so, when you run the designer tool to generate a migration

cumslvt13OP•13mo ago

and those are actually generated from models + OnModelCreating

JakenVeina•13mo ago

it calls that BuildModel() method to generate a model, in memory, which represents (in thoery) the current state of the physical database and it does the same thing for DbContext.OnModelCreating() to generate a model, in memory, of whta you WANT the database to be then it can effectively diff those two object structures that's where the migration code comes from

cumslvt13OP•13mo ago

Understood

JakenVeina•13mo ago

that file also gives you, as the developer, the chance to modify it, if you have anything to model that EF doesn't properly support same for the migration code itself

cumslvt13OP•13mo ago

Guess my question is answered Thanks again for the explanation Remind me, how to properly resolve the question here There was a command or something

JakenVeina•13mo ago

uhhhhhhhhhhhhhhhh

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✅ What's the point of ModelSnapshot in ef core migrations