Unique Constraint in Entity Framework Code First

Question

Question

Is it possible to define a unique constraint on a property using either the fluent syntax or an attribute? If not, what are the workarounds?

I have a user class with a primary key, but I would like to make sure the email address is also unique. Is this possible without editing the database directly?

Solution (based on Matt's answer)

public class MyContext : DbContext {
    public DbSet<User> Users { get; set; }

    public override int SaveChanges() {
        foreach (var item in ChangeTracker.Entries<IModel>())
            item.Entity.Modified = DateTime.Now;

        return base.SaveChanges();
    }

    public class Initializer : IDatabaseInitializer<MyContext> {
        public void InitializeDatabase(MyContext context) {
            if (context.Database.Exists() && !context.Database.CompatibleWithModel(false))
                context.Database.Delete();

            if (!context.Database.Exists()) {
                context.Database.Create();
                context.Database.ExecuteSqlCommand("alter table Users add constraint UniqueUserEmail unique (Email)");
            }
        }
    }
}

Solution 1

As far as I can tell, there's no way to do this with Entity Framework at the moment. However, this isn't just a problem with unique constraints... you may want to create indexes, check constraints, and possibly triggers and other constructs too. Here's a simple pattern you can use with your code-first setup, though admittedly it's not database agnostic:

public class MyRepository : DbContext {
    public DbSet<Whatever> Whatevers { get; set; }

    public class Initializer : IDatabaseInitializer<MyRepository> {
        public void InitializeDatabase(MyRepository context) {
            if (!context.Database.Exists() || !context.Database.ModelMatchesDatabase()) {
                context.Database.DeleteIfExists();
                context.Database.Create();

                context.ObjectContext.ExecuteStoreCommand("CREATE UNIQUE CONSTRAINT...");
                context.ObjectContext.ExecuteStoreCommand("CREATE INDEX...");
                context.ObjectContext.ExecuteStoreCommand("ETC...");
            }
        }
    }
}

Another option is if your domain model is the only method of inserting/updating data in your database, you could implement the uniqueness requirement yourself and leave the database out of it. This is a more portable solution and forces you to be clear about your business rules in your code, but leaves your database open to invalid data getting back-doored.

Solution 2

Starting with EF 6.1 it is now possible:

[Index(IsUnique = true)]
public string EmailAddress { get; set; }

This will get you a unique index instead of unique constraint, strictly speaking. For most practical purposes they are the same.

Solution 3

Not really related to this but it might help in some cases.

If you're looking to create a unique composite index on let's say 2 columns that will act as a constraint for your table, then as of version 4.3 you can use the new migrations mechanism to achieve it:

• http://msdn.microsoft.com/en-us/library/hh770484(v=vs.103).aspx
• http://blogs.msdn.com/b/adonet/archive/2012/02/09/ef-4-3-code-based-migrations-walkthrough.aspx

Basically you need to insert a call like this in one of your migration scripts:

CreateIndex("TableName", new string[2] { "Column1", "Column2" }, true, "IX_UniqueColumn1AndColumn2");

Something like that:

namespace Sample.Migrations
{
    using System;
    using System.Data.Entity.Migrations;

    public partial class TableName_SetUniqueCompositeIndex : DbMigration
    {
        public override void Up()
        {
            CreateIndex("TableName", new[] { "Column1", "Column2" }, true, "IX_UniqueColumn1AndColumn2");
        }

        public override void Down()
        {
            DropIndex("TableName", new[] { "Column1", "Column2" });
        }
    }
}

Solution 4

Just trying to find out if there was a way to do this, only way I found so far was enforcing it myself, I created an attribute to be added to each class where you supply the name of the fields you need to be unique:

    [System.AttributeUsage(System.AttributeTargets.Class, AllowMultiple=false,Inherited=true)]
public class UniqueAttribute:System.Attribute
{
    private string[] _atts;
    public string[] KeyFields
    {
        get
        {
            return _atts;
        }
    }
    public UniqueAttribute(string keyFields)
    {
        this._atts = keyFields.Split(new char[]{','}, StringSplitOptions.RemoveEmptyEntries);
    }
}

Then in my class I'll add it:

[CustomAttributes.Unique("Name")]
public class Item: BasePOCO
{
    public string Name{get;set;}
    [StringLength(250)]
    public string Description { get; set; }
    [Required]
    public String Category { get; set; }
    [Required]
    public string UOM { get; set; }
    [Required]
}

Finally, I'll add a method in my repository, in the Add method or when Saving Changes like this:

private void ValidateDuplicatedKeys(T entity)
{
    var atts = typeof(T).GetCustomAttributes(typeof(UniqueAttribute), true);
    if (atts == null || atts.Count() < 1)
    {
        return;
    }
    foreach (var att in atts)
    {
        UniqueAttribute uniqueAtt = (UniqueAttribute)att;
        var newkeyValues = from pi in entity.GetType().GetProperties()
                            join k in uniqueAtt.KeyFields on pi.Name equals k
                            select new { KeyField = k, Value = pi.GetValue(entity, null).ToString() };
        foreach (var item in _objectSet)
        {
            var keyValues = from pi in item.GetType().GetProperties()
                            join k in uniqueAtt.KeyFields on pi.Name equals k
                            select new { KeyField = k, Value = pi.GetValue(item, null).ToString() };
            var exists = keyValues.SequenceEqual(newkeyValues);
            if (exists)
            {
                throw new System.Exception("Duplicated Entry found");
            }
        }
    }
}

Not too nice as we need to rely on reflection but this so far is the approach that works for me! =D

Solution 5

Also in 6.1 you can use the fluent syntax version of @mihkelmuur's answer like so:

Property(s => s.EmailAddress).HasColumnAnnotation(IndexAnnotation.AnnotationName,
new IndexAnnotation(
    new IndexAttribute("IX_UniqueEmail") { IsUnique = true }));

The fluent method isnt perfect IMO but at least its possible now.

More deets on Arthur Vickers blog http://blog.oneunicorn.com/2014/02/15/ef-6-1-creating-indexes-with-indexattribute/

View more solutions

Comments

[kim3er]

I like my DB to be as tight as a drum, the logic is replicated in the business layer. You're answer only works with CTP4 but got me on the right track, I've provided a solution that is compatible with CTP5 below my original question. Thanks a lot!

[kim3er]

Thanks Kelly! I wasn't aware of that event handler. My eventual solution places the SQL in the InitializeDatabase method.

[devuxer]

Unless your app is single-user, I believe a unique constraint is one thing you can't enforce with code alone. You can dramatically reduce the probability of a violation in code (by checking uniqueness prior to calling SaveChanges()), but there's still the possibility of another insert/update slipping in between the time of the uniqueness check and the time of SaveChanges(). So, depending on how mission critical the app is and the liklihood of a uniqueness violation, it's probably best to add the constraint to the database.

[mattmc3]

You'd have to have your check for uniqueness be part of the same transaction as your SaveChanges. Assuming your database is acid compliant you should absolutely be able to enforce uniqueness this way. Now whether EF allows you to properly manage the transaction lifecycle this way is another question.

[kim3er]

Nice to see EF have got Rails style migrations. Now if only I could run it on Mono.

[Daryn]

I have tried this, but the line !context.Database.CompatibleWithModel(true) is never true. This line seems to happen after the migrations have run, therefore the database is compatible with the model. Might this be because my the dbcontext class uses public AppDatabaseContext() : base("ConStringName")? I set up the initializer in the constructor of AppDatabaseContext, is that correct?

[Daryn]

Is there any reason why we can't simply use the SQL function in the migration file, e.g: public override void Up() { AddColumn("Posts", "Abstract", c => c.String()); Sql("UPDATE Posts SET Abstract = LEFT(Content, 100) WHERE Abstract IS NULL"); } - using the correct SQL. - msdn.microsoft.com/en-US/data/jj591621

[Michael Bisbjerg]

Shouldn't you also have a DropIndex in the Down() procedure? DropIndex("TableName", new[] { "Column1", "Column2" });

[tandrewnichols]

@Daryn - You can do that, but if you ever need to alter or recreate your migration file, any manual changes get overwritten.

[Alex]

You can absolutely do this with Entity Framework, using the ValidateEntity method. Answer

[Shaun Wilson]

This is actually a more appropriate answer than a custom DB initializer.

[user2246674]

@DanM That's why transactions exist. When you need such serialized guarantees.

[Nathan]

@mattmc3 It depends on your transaction isolation level. Only the serializable isolation level (or custom table locking, ugh) actually would allow you to guarantee uniqueness in your code. But most people don't use the serializable isolation level because of performance reasons. The default in MS Sql Server is read committed. See the 4 part series starting at: michaeljswart.com/2010/03/…

[sotn]

EntityFramework 6.1.0 has support for IndexAttribute now which you can basically add it on top of the properties.

[Mihkel Müür]

@Dave: just use the same index name on the attributes of the respective properties (source).

[Richard]

Note this creates a unique index rather than a unique contraint. While almost the same they are not quite the same (as I understand it unique constrains can be used as the target of an FK). For a constraint you need to execute SQL.

[Richard]

(Following the last comment) Other sources suggest this limitation has been removed in more recent versions of SQL Server... but BOL is not completely consistent.

[Mihkel Müür]

@Richard: attribute-based unique constraints are also possible (see my second answer), though not out of the box.

[exSnake]

Is it possible to define a unique or null constrain? I mean, it should be null if setted, via data annotations? Because i'm working with code first migration but if i define a IsUnique Index if it's not setted the new colum will return an error

[Mihkel Müür]

@exSnake: Since SQL Server 2008, unique index supports a single NULL value per column by default. In case support for multiple NULLs is required, a filtered index would be needed see another question.

[exSnake]

@MihkelMüür What you mean with filtered index? How i can create a filtered index with Code First workflow and migrations in VS2018 without editing the migration code manually?

[Mihkel Müür]

@exSnake: filtered index uses a filter predicate (the WHERE-clause) and only applies to the matching subset of the rows. Not sure if it is supported by Code First approach though.

[Benjineer]

Maybe give some context or code rather than just a URL