Sitecore Experience Commerce - The entity does not have a key defined

After creating a custom command, API and return entity, I was getting the following error when starting the commerce engine in debug mode.

The entity 'MyModel' does not have a key defined
at Microsoft.AspNetCore.OData.Builder.ODataModelBuilder.ValidateModel(IEdmModel model)

The fix was to add the [Key] attribute to one of the attributes of my model. This can be found under the System.ComponentModel.DataAnnotations model.

public class ExtendedPricingDetail
{
 [Key]
 public string ProductId { get; set; }
 public Decimal CurrentPrice { get; set; }
 public Decimal OriginalPrice { get; set; }
 public string PromotionCode { get; set; }
 public string PromotionText { get; set; }
 public bool OnSpecial { get; set; }

 public ExtendedPricingDetail()
 {
  ProductId = string.Empty;
  CurrentPrice = 0.00M;
  OriginalPrice = 0.00M;
  PromotionCode = string.Empty;
  PromotionText = string.Empty;
  OnSpecial = false;
 }
}