How to handle field level Acces Security when using Entity Framework

I have similar problem: I have several users with different permissions. Some of them can do everything and some of them can select/update only certain columns in tables. The solution I have found is the using of OnModelCreating method.

Now I'd like to explain the general logic. I have database with users who are authenticated by SQL login. These users are mapped to one of custom database roles, and these roles, in their turn, are granted permissions. If you use Windows Authentication, there's really no difference for code since the idea would be the same: you would create Windows Groups, map your users to those groups and, finally, grant permissions to groups.

The core of this idea is Ignore method of EntityTypeConfiguration<T> used in OnModelCreating method (which you override in your custom context class). This method lets us ignore the columns in entities which won't be used by context. So, you should set some condition which branches to code with filtering:

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    // Get the entity to which we want to apply filter
    var detailEntity = modelBuilder.Entity<Detail>();

    // Check the condtion
    if (someCondition)
    {
        // If condition is met, these two properties will be ignored by context.
        // From now on, these properties will get their default values
        // when selecting data from database:
        // for numeric types - zero;
        // for reference types - null.
        detailEntity.Ignore(entity => entity.Price);
        detailEntity.Ignore(entity => entity.Discount);
    }
}

The condition for filtering is the name of Database Role in SQL Server (if you use SQL Login) or Windows Group the user belongs to (if you use Windows Authentication).

1. Windows Groups

For Windows Groups it's easy - just fetch all groups the user belongs to and compare it to the one you need. Here's helper class which would fetch user's Windows Groups and check whether he belongs to the group passed as parameter:

static class User
{
    internal static bool IsInGroup(string groupName)
    {
        return GetWindowsGroups().Any(g => g.ToLower() == groupName.ToLower());
    }

    internal static List<string> GetWindowsGroups()
    {
        List<string> groups = new List<string>();
        WindowsIdentity user = WindowsIdentity.GetCurrent();
        user.Groups.ToList().ForEach(ir =>
        {
            try
            {
                IdentityReference irt = ir.Translate(typeof(NTAccount));
                groups.Add(irt.Value);
            }
            catch { /* just ignore */ }
        });
        return groups;
    }
}

Now we can use it in OnModelCreating:

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    var detailEntity = modelBuilder.Entity<Detail>();
    if (User.IsInGroup("DB_OPERATOR"))
    {
        // Fully ignore properties in model
        detailEntity.Ignore(entity => entity.Price);
        detailEntity.Ignore(entity => entity.Discount);
    }
}

Now any selection from Detail table will not select these two properties (although, as I said earlier, you can use them in your entity class - they just will be initialized to their default values, but changes to them won't propagate to database).

2. SQL Logins

With SQL Logins, the user's permissions depend on his database role, so you have to obtain it before any interaction with context. Here's where things become a bit complicated. Before going further, first, I will use dbo.GetUserRole stored procedure which just fetches a single value - role a user belongs to:

CREATE PROCEDURE dbo.GetUserRole
AS
BEGIN
    SET NOCOUNT ON;
    SELECT dp.[name] --, us.[name]
    FROM sys.sysusers AS us
    RIGHT JOIN sys.database_role_members AS rm ON us.uid = rm.member_principal_id
          JOIN sys.database_principals AS dp ON rm.role_principal_id =  dp.principal_id
    WHERE us.name = CURRENT_USER;
END;

Second, I will add static Role property indicating user's database role:

class TestContext : DbContext
{
    internal static string Role { get; private set; }
}

Now there's one caveat that awaits us: You can't fetch role inside OnModelCreating method.

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    Role = base.Database.SqlQuery<string>("dbo.GetUserRole", new object[] { }).First();
    var detailEntity = modelBuilder.Entity<Detail>();
    if (Role == "operator")
    {
        detailEntity.Ignore(entity => entity.Price);
        detailEntity.Ignore(entity => entity.Discount);
    }
}

Just because EF will throw an exception:

The context cannot be used while the model is being created. This exception may be thrown if the context is used inside the OnModelCreating method or if the same context instance is accessed by multiple threads concurrently. Note that instance members of DbContext and related classes are not guaranteed to be thread safe.

Well, if you can't use context, then we can use context's DbConnection directly from constructor:

class TestContext : DbContext
{
    public DbSet<Detail> Details { get; set; }
    internal static string Role { get; private set; }

    public TestContext(string nameOrConnectionString) : base(nameOrConnectionString)
    {
        var conn = base.Database.Connection;
        conn.Open();
        using (var comm = conn.CreateCommand())
        {
            comm.CommandText = "dbo.GetUserRole";
            comm.CommandType = CommandType.StoredProcedure;
            Role = comm.ExecuteScalar() as string;
        }
        conn.Close();
    }
}

So, for now we have the following. The OnModelCreating is called in one of two cases:

1) When you call DbContext.Database.Initialize method.

2) When you make a query against database (in this case Database.Initialize runs implicitly).

As a side note, the documentation says:

This method is called only once when the first instance of a derived context is created.

However, this is not true.

So, the solution is to use context's connection and request a role. This:

1) must be done before any query or Database.Initialize call

or

2) can be done in context's constructor.

Now when we have a role, we can use it in our code. I chose String type for storing a role, but you can also use enumeration for convenience:

enum Role
{
    Admin,
    Regular,
    Operator
}

I don't know whether this pattern I chose is correct, but I haven't found any info over this theme - and I'm surprised, because in reality this is how databases are designed in mind - with granting different permissions to different users. So, I hope this will help you.

Good luck!