How to Resolve Entity Key Conflicts When Editing Entities in ASP.NET MVC?

Troubleshooting Entity Key Conflicts in ASP.NET MVC Entity Updates

When updating entities in ASP.NET MVC via POST requests, you might encounter the following error:

<code>Attaching an entity of type 'MODELNAME' failed because another entity of the same type already has the same primary key value.</code>

This error arises from attempting to attach an entity to the DbContext while another entity with the identical primary key already exists within the context's object graph. The DbContext tries to merge the entities, resulting in a conflict.

Scenario and Root Cause

Imagine editing an 'A' entity within a controller. The GET action uses Find() to load the entity. The POST action then attempts to attach and update it. However, if another 'A' entity with the same primary key is loaded elsewhere (e.g., during user access validation), this creates a tracking conflict.

Resolution using AsNoTracking()

The most effective solution is to prevent the DbContext from tracking the entity used for validation. Use the AsNoTracking() method:

private bool canUserAccessA(int aID)
{
    int userID = WebSecurity.GetUserId(User.Identity.Name);
    int aFound = db.Model.AsNoTracking().Where(x => x.aID == aID && x.UserID == userID).Count();
    return (aFound > 0);
}

AsNoTracking() ensures the validation entity doesn't interfere with the entity being updated.

Alternative: The Detach() Method

Another approach involves detaching the entity from the DbContext before modification using the Detach() method. This prevents tracking conflicts. However, be aware that detaching might impact other parts of your code, so carefully consider its implications before implementing this solution.

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