efCoreCompositeKey

Solution for using data annotation composite keys with EntityFramework Core.

EntityFramework Core does not allow to use System.ComponentModel.DataAnnotations.KeyAttribute with multiple columns to define composite key. I dont know the reason of this variant of feature implementation, but i have some projects using EntityFramework6 and there are some entities with composite keys defined as property attributes. Since some of the projects are multi-platformed (.Net 4.5 and .Net Standard 2.0) and they are using common class library with entities definitions (this library is dont know about EF versions) i cannot use Fluent API in data contexts.

This solution solves my problem completely and now i can use old data annotation definitions with EF Core. Maybe i lose some of the important moments, but the fixes and issues are welcome.

Solution is with directly redefining primary key for entity types with comples keys. This is based on code from KeyAttributeConvention class.

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            foreach (var entityTypeFirst in modelBuilder.Model.GetEntityTypes())
            {
                if (entityTypeFirst is EntityType entityType)
                {
                    if (entityType.BaseType == null)
                    {
                        var primaryKey = entityType.FindPrimaryKey();
                        if ((primaryKey != null ? (primaryKey.Properties.Count > 1 ? 1 : 0) : 0) != 0)
                        {
                            if (entityType.GetPrimaryKeyConfigurationSource() == ConfigurationSource.DataAnnotation)
                            {
                                var keyProperties = entityType.GetProperties().Where(x => x.IsKey() && x.GetConfigurationSource() != ConfigurationSource.Convention).ToList();
                                entityType.SetPrimaryKey(keyProperties, ConfigurationSource.Explicit);
                            }
                        }
                    }
                    else
                    {
                        foreach (Property declaredProperty in entityType.GetDeclaredProperties())
                        {
                            MemberInfo identifyingMemberInfo = declaredProperty.GetIdentifyingMemberInfo();
                            if (identifyingMemberInfo != (MemberInfo)null && Attribute.IsDefined(identifyingMemberInfo, typeof(KeyAttribute), true))
                            {
                                throw new NotImplementedException("EF Core hack with replacing data annotation primary keys - not implemented fully.");
                            }
                        }
                    }
                }
            }
        }