I'm adding an Entity (Category) as a property in another Entity (Product). System.ArgumentException: An item with the same key has already been added

Question

In my API project, there is the entity Category as below:

    [Key]
    public string Id { get; set; }                
    public string Title { get; set; }
    public Category(string id, string title)

And the Entity Product where Category is a property:

    [Key]
    public string Id { get; set; }

    [Required(ErrorMessage = "Título é obrigatório.")]
    [MaxLength(60, ErrorMessage = "Este campo deve conter enre 3 e 60 caracteres.")]
    [MinLength(3, ErrorMessage = "Este campo deve conter enre 3 e 60 caracteres.")]
    public string Title { get; set; }

    [MaxLength(1024, ErrorMessage = "Este campo deve ter no máximo 1024 caracteres")]
    public string Description { get; set; }

    [Required(ErrorMessage = "Este campo é obrigatório")]
    [Range(1, int.MaxValue, ErrorMessage = "O preço deve ser maior que zero")]
    public decimal Price { get; set; }
    public int AvailableQuantity { get; set; }

    [Required(ErrorMessage = "Este campo é obrigatório")]
    public string CategoryId { get; set; }

    public Category Category { get; set; }

This is the Create Method in the ProductController:

    [HttpPost]
    [AllowAnonymous]
    public async Task<ActionResult<Product>> Create([FromBody] Product model)
    {
        var category = _context.Categories.AsNoTracking().FirstOrDefault(x => x.Id == model.CategoryId);

        if (category == null)
            return Ok("Categoria informada não existe");


        if (ModelState.IsValid)
        {
            _product.GenerateId(model);
            var product = new Product(model.Id, model.Title, model.Description, model.Price, model.CategoryId, category);
            _context.Products.Add(product);
            await _context.SaveChangesAsync();
            return product;
        }
        else
        {
            return BadRequest(ModelState);
        }
    }

When I try to create a new Product I receive the error message as below:

System.ArgumentException: An item with the same key has already been added. Key: 280CF21D at System.Collections.Generic.Dictionary2.TryInsert(TKey key, TValue value, InsertionBehavior behavior) at System.Collections.Generic.Dictionary2.Add(TKey key, TValue value) at Microsoft.EntityFrameworkCore.InMemory.Storage.Internal.InMemoryTable1.Create(IUpdateEntry entry) at Microsoft.EntityFrameworkCore.InMemory.Storage.Internal.InMemoryStore.ExecuteTransaction(IList1 entries, IDiagnosticsLogger1 updateLogger) at Microsoft.EntityFrameworkCore.InMemory.Storage.Internal.InMemoryDatabase.SaveChangesAsync(IList1 entries, CancellationToken cancellationToken) at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.SaveChangesAsync(IList1 entriesToSave, CancellationToken cancellationToken) at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.SaveChangesAsync(DbContext _, Boolean acceptAllChangesOnSuccess, CancellationToken cancellationToken) at Microsoft.EntityFrameworkCore.DbContext.SaveChangesAsync(Boolean acceptAllChangesOnSuccess, CancellationToken cancellationToken) at Microsoft.EntityFrameworkCore.DbContext.SaveChangesAsync(Boolean acceptAllChangesOnSuccess, CancellationToken cancellationToken) at TesteAPI.Controllers.ProductsController.Create(Product model) in C:\Users\tcho3\source\repos\TesteAPI\TesteAPI\Controllers\ProductsController.cs:line 93 at lambda_method56(Closure , Object ) at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.AwaitableObjectResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments) at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeActionMethodAsync>g__Awaited|12_0(ControllerActionInvoker invoker, ValueTask1 actionResultValueTask) at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.g__Awaited|10_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted) at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Rethrow(ActionExecutedContextSealed context) at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted) at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeInnerFilterAsync() --- End of stack trace from previous location --- at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.g__Awaited|19_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted) at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.g__Awaited|17_0(ResourceInvoker invoker, Task task, IDisposable scope) at Microsoft.AspNetCore.Routing.EndpointMiddleware.g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger) at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context) at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context) at Swashbuckle.AspNetCore.SwaggerUI.SwaggerUIMiddleware.Invoke(HttpContext httpContext) at Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.Invoke(HttpContext httpContext, ISwaggerProvider swaggerProvider) at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

see: public void GenerateId(Product product) { product.Id = Guid.NewGuid().ToString().Replace("-", "").ToUpper().Substring(0, 8); } — Gustavo Kaic Corrêa

Serge Serge · Accepted Answer · 2021-07-05T14:26:07

fix model

public class Category 
{
    [Key]
    public string Id { get; set; }                
    public string Title { get; set; }
    .....

   public virtual ICollection<Product> Products { get; set}
}

You don't need a special constructor for Product too since you are using Product as a model

fix your action


  public async Task<ActionResult<Product>> Create([FromBody] Product model)
    {
     
//---------- I don't think that you really need this

  var category = _context.Categories.AsNoTracking().FirstOrDefault(x => x.Id == model.CategoryId);
        if (category == null)  return Ok("Categoria informada não existe");

//---------

 //would be enough
    if (model.category == 0)  return Ok("Categoria informada não existe");



        if (ModelState.IsValid)
        {
            _product.GenerateId(model);
            _context.Products.Add(model;
            await _context.SaveChangesAsync();
            return model;
        }
        else
        {
            .....
        }
    }

I'm adding an Entity (Category) as a property in another Entity (Product). System.ArgumentException: An item with the same key has already been added

2 Answers