C#•3d ago

✅ Seeding question

Hi all, In my app I am using EF Core I have a many to many relationship between machine and option Every machine has a different price per option, so I added price as payload to the junction table. Pseudocode: Machine:

List<OptionMachine> options; // Only holding OptionMachine list and not of options because we only want to access the options with a price.
String description;

List<OptionMachine> options; // Only holding OptionMachine list and not of options because we only want to access the options with a price.
String description;

Option:

List<Machine> machines;

List<Machine> machines;

OptionMachine:

    public int MachineId { get; set; } = default!;

    private Machine machine = default!;
    public required Machine Machine
    {
        get => machine;
        set => machine = value ?? throw new ArgumentNullException();
    }


    public int OptionId { get; set; } = default!;

    private Option Option = default!;
    public required =Option Option
    {
        get => customOption;
        set => customOption = value ?? throw new ArgumentNullException();
    }


    private decimal price = default!;

    public required decimal Price
    {
        get;
        set;
    }

    public int MachineId { get; set; } = default!;

    private Machine machine = default!;
    public required Machine Machine
    {
        get => machine;
        set => machine = value ?? throw new ArgumentNullException();
    }


    public int OptionId { get; set; } = default!;

    private Option Option = default!;
    public required =Option Option
    {
        get => customOption;
        set => customOption = value ?? throw new ArgumentNullException();
    }


    private decimal price = default!;

    public required decimal Price
    {
        get;
        set;
    }

Now my question is how do I seed Machine? Lets say I have some existing options:

var options = await dbContext.Options.ToListAsync();

//Machine to seed:
Machine machine = new(){
  description="MyTestMachine",
  options = { new(){ Machine = //how to reference machine here?, 
                    Option= options[0], price=500M }}
  
}

var options = await dbContext.Options.ToListAsync();

//Machine to seed:
Machine machine = new(){
  description="MyTestMachine",
  options = { new(){ Machine = //how to reference machine here?, 
                    Option= options[0], price=500M }}
  
}

As you can see I cannot use the machine itself to seed the junction table, how do I go about doing this?

16 Replies

Jiry_XDOP•3d ago

I know I can probably add them later, outside of the initialization but I'd like to enforce that a machine needs to exist WITH options WITH a price.

Keswiik•2d ago

You do not have to set Machine in your OptionMachine entity for this to work. Assuming your relationships aren't messed up, EF should handle this for you. https://learn.microsoft.com/en-us/ef/core/saving/related-data#adding-a-graph-of-new-entities I also don't know why you're not using auto props here

public class OptionMachine {
  [ForeignKey("Machine")]
  public int MachineId { get; set; } = default;
  public Machine? Machine { get; set; } # nullable because you can load the junction entity without loading the nav props

  [ForeignKey("Option")]
  public int OptionId { get; set; } = default;
  public Option? Option { get; set; }

  [Required]
  public decimal Price { get; set; } = default;
}

public class OptionMachine {
  [ForeignKey("Machine")]
  public int MachineId { get; set; } = default;
  public Machine? Machine { get; set; } # nullable because you can load the junction entity without loading the nav props

  [ForeignKey("Option")]
  public int OptionId { get; set; } = default;
  public Option? Option { get; set; }

  [Required]
  public decimal Price { get; set; } = default;
}

Jiry_XDOP•2d ago

Gotcha thanks! One more question however: As you can see in my machine class I have only a List of OptionMachine But not a list of Option. Online I always saw that they add both the explicit Junction/Join entity and the entity itself?: https://learn.microsoft.com/en-us/ef/core/modeling/relationships/many-to-many#many-to-many-and-join-table-with-payload https://stackoverflow.com/questions/78766403/net-ef-core-how-to-correctly-insert-and-update-a-model-in-a-many-to-many-rela For example in above Post, they hold both a list of Tag and a list of PostTag

Stack Overflow

.NET EF Core : how to correctly insert and update a model in a many...

I am writing a .NET CRUD application with layered architecture with EF Core for ORM. I have two models (Players and Teams) stored in the database with a join table between them (Player in Team). Th...

Many-to-many relationships - EF Core

How to configure many-to-many relationships between entity types when using Entity Framework Core

Jiry_XDOP•2d ago

Do you need both or is one fine?

Keswiik•2d ago

Not sure, I always set my many-many relationships up to use an explicit join table

Jiry_XDOP•2d ago

Okay thanks, do you use them both? A list of your item and a list of your junction table? For example this Player class has both, but I am not sure if its necesarry?:

Keswiik•2d ago

No, I only use the explicit join table in my entities There are probably cases where having access to the other nav collection would be useful, but I don't use it myself

Jiry_XDOP•2d ago

Gotcha, so you only hold TeamPlayers in the image's case?

Keswiik•2d ago

Yup

Jiry_XDOP•2d ago

Great thanks, that's what I was wondering if it worked Thanks 😛

Keswiik•2d ago

:PepoSalute:

Jiry_XDOP•2d ago

True, thats why I wanted to omit it

Keswiik•2d ago

one thing to note, though, if you are creating a new entity and it references other entities that already exist, you may only need to set the ID on your join entity say, for example, you are creating a new machine that references an existing option i would probably do something like

var newMachine = new Machine {
  // stuff here
  Options = new() {
    new() {
      OptionId = someExistingOption.Id
    }
  }
};

var newMachine = new Machine {
  // stuff here
  Options = new() {
    new() {
      OptionId = someExistingOption.Id
    }
  }
};

I've had times where the EF change tracker tried re-creating the entities specified if the nav property was set to something that already existed

Jiry_XDOP•2d ago

Oh I see thanks for the tip 🙂 I'll put that to good use 😛

Keswiik•2d ago

:PepoSalute: gl

Jiry_XDOP•2d ago

Thanks 😋

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✅ Seeding question