How to Resolve 'Attaching an Entity Failed Due to Duplicate Primary Key Values' Errors in ASP.NET MVC?

Troubleshooting Duplicate Primary Key Errors When Attaching Entities in ASP.NET MVC

In ASP.NET MVC applications, attempting to attach an entity can fail due to a duplicate primary key constraint violation. This often manifests as an error message indicating that an entity with the same primary key already exists. This article details a common cause and its solution.

The problem frequently arises when updating an entity's state to EntityState.Modified using db.Entry(entity).State = EntityState.Modified. Investigation often reveals that a separate function, responsible for authorization or data retrieval, pre-loads the same entity before the state update. This pre-loading process can lead to the entity becoming detached, causing the subsequent Attach operation to fail because the framework believes it's trying to add a duplicate.

The solution involves preventing the pre-loading function from tracking the loaded entity. This can be achieved by utilizing the AsNoTracking() method in the Entity Framework query.

Consider the following example of a problematic canUserAccessA function:

// Problematic function - loads and tracks the entity
private bool canUserAccessA(int aID)
{
    int userID = WebSecurity.GetUserId(User.Identity.Name);
    int aFound = db.Model.Where(x => x.aID == aID && x.UserID == userID).Count(); // Tracks the entity

    return (aFound > 0);
}

This function loads the entity, making it trackable by the context. To correct this, modify the function to use AsNoTracking():

// Corrected function - loads the entity without tracking
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);
}

Using AsNoTracking() ensures that the loaded entity is not tracked by the context, preventing conflicts with subsequent Attach or Update operations. Note that while using .Find(aID) with AsNoTracking might seem simpler, it may be ineffective depending on the database and context configuration. The provided query offers a more robust approach.

This modification effectively resolves the duplicate primary key issue, providing a reliable solution for similar problems in ASP.NET MVC development.

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