How Can I Prevent Entity Framework from Persisting Child Objects?

Suppressing Child Object Persistence in Entity Framework

In Entity Framework, it is generally assumed that saving an entity will automatically persist its child objects as well. However, in certain scenarios, it may be desirable to prevent this behavior. This is particularly useful when dealing with flattened data that may not correspond directly to the database schema.

Let's consider a hypothetical scenario where a School entity contains a City property. While populating the School object from a flat file, the City property may reference lookup tables to determine geographic locations. However, since these city definitions already exist in the database, it is not necessary for Entity Framework to insert them again when saving the School.

To prevent Entity Framework from persisting child objects in such situations, there are two approaches:

Approach 1: Using EntityState

One approach involves manually setting the EntityState of the child object to Unchanged. This instructs Entity Framework to ignore the child object during the save operation. Here's an example:

using (var context = new DatabaseContext())
{
    context.Set<School>().Add(newItem);
    // Set the City property to Unchanged
    context.Entry(newItem.City).State = EntityState.Unchanged;
    context.SaveChanges();
}

Approach 2: Using Foreign Key

A more elegant and flexible approach is to utilize the Foreign Key property in your entity definition. Define a Foreign Key attribute on the child property to explicitly specify its relationship to the foreign entity. For example:

public class School
{
    [ForeignKey("City_Id")]
    public City City { get; set; }
    [Required]
    public int City_Id { get; set; }
}

This approach allows you to specify the foreign key value explicitly during object creation, informing Entity Framework that the child object has already been persisted. The City property can be omitted from the context graph by setting it to null when saving:

newItem.City = null;
context.Set<School>().Add(newItem);
context.SaveChanges();

By employing either of these approaches, you can prevent Entity Framework from attempting to save child objects, ensuring that only the desired entities are persisted to the database.

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