Repository pattern and mapping between domain models and Entity Framework

With entity framework, across all layers IMHO, it is generally a bad idea to convert from entity models to other form of models (domain models, value objects, view models, etc.) and vice versa except only in the application layer because you will lose a lot of EF capabilities that you can only achieve through the entity objects, like loss of change tracking and loss of LINQ queryable.

It's better to do the mapping between your repository layer and the application layer. Keep the entity models in the repository layer.

My repositories deal with and provide persistence for a rich domain model. I do not want to expose the anemic, Entity Framework data entity to my business layers, so I need some way of mapping between them.

If you you use Entity Framework, it can map Rich Domain Model itself.

I've answered the similar question "Advice on mapping of entities to domain objects" recently.

I've been using NHibernate and know that in Entity Framework you can also specify mapping rules from DB tables to your POCO objects. It is an extra effort to develop another abstraction layer over Entity Framework entities. Let the ORM be responsible for all of the mappings, state tracking, unit of work and identity map implementation, etc. Modern ORMs know how to handle all these issues.

AutoMapper could be used for the opposite situation (mapping to data entities) but not when creating domain models.

You are completely right.

Automapper is useful when one entity can be mapped into another without additional dependencies (e.g. Repositories, Services, ...).

... where would I define the mapping between a Department and a DepartmentDataEntity?

I would put it into DepartmentRepository and add method IList<Department> FindByCompany(int companyId) in order to retreive company's departments.

I could provide more mapping methods in the CompanyRepository, but this will soon get messy. There would soon be duplicated mapping methods across the system.

What is a better approach to the above problem?

If it is needed to get list of Departments for another entity, a new method should be added to DepartmentRepository and simply used where it is needed.

I like using custom-built Extension methods to do the mapping between Entity and Domain objects.

• You can easily call extension methods for other entities when they are within a containing entity.
• You can easily deal with collections by creating IEnumerable<> extensions.

A simple example:

public static class LevelTypeItemMapping
{
    public static LevelTypeModel ToModel(this LevelTypeItem entity)
    {
        if (entity == null) return new LevelTypeModel();

        return new LevelTypeModel
        { 
            Id = entity.Id;
            IsDataCaptureRequired = entity.IsDataCaptureRequired;
            IsSetupRequired = entity.IsSetupRequired;
            IsApprover = entity.IsApprover;
            Name = entity.Name;
        }
    }
    public static IEnumerable<LevelTypeModel> ToModel(this IEnumerable<LevelTypeItem> entities)
    {
        if (entities== null) return new List<LevelTypeModel>();

        return (from e in entities
                select e.ToModel());
    }

}

...and you use them like this.....

 using (IUnitOfWork uow = new NHUnitOfWork())
        {
        IReadOnlyRepository<LevelTypeItem> repository = new NHRepository<LevelTypeItem>(uow);
        record = repository.Get(id);

        return record.ToModel();

        records = repository.GetAll(); // Return a collection from the DB
        return records.ToModel(); // Convert a collection of entities to a collection of models
        }

Not perfect, but very easy to follow and very easy to reuse.

Let's say you have the following data access object...

public class AssetDA
{        
    public HistoryLogEntry GetHistoryRecord(int id)
    {
        HistoryLogEntry record = new HistoryLogEntry();

        using (IUnitOfWork uow = new NHUnitOfWork())
        {
            IReadOnlyRepository<HistoryLogEntry> repository = new NHRepository<HistoryLogEntry>(uow);
            record = repository.Get(id);
        }

        return record;
    }
}

which returns a history log entry data entity. This data entity is defined as follows...

public class HistoryLogEntry : IEntity
{
    public virtual int Id
    { get; set; }

    public virtual int AssetID 
    { get; set; }

    public virtual DateTime Date
    { get; set; }

    public virtual string Text
    { get; set; }

    public virtual Guid UserID
    { get; set; }

    public virtual IList<AssetHistoryDetail> Details { get; set; }
}

You can see that the property Details references another data entity AssetHistoryDetail. Now, in my project I need to map these data entities to Domain model objects which are used in my business logic. To do the mapping I have defined extension methods...I know it's an anti-pattern since it is language specific, but the good thing is that it isolate and breaks dependencies between each other...yeah, that's the beauty of it. So, the mapper is defined as follows...

internal static class AssetPOMapper
{
    internal static HistoryEntryPO FromDataObject(this HistoryLogEntry t)
    {
        return t == null ? null :
            new HistoryEntryPO()
            {
                Id = t.Id,
                AssetID = t.AssetID,
                Date = t.Date,
                Text = t.Text,
                UserID = t.UserID,
                Details = t.Details.Select(x=>x.FromDataObject()).ToList()
            };
    }

    internal static AssetHistoryDetailPO FromDataObject(this AssetHistoryDetail t)
    {
        return t == null ? null :
            new AssetHistoryDetailPO()
            {
                Id = t.Id,
                ChangedDetail = t.ChangedDetail,
                OldValue = t.OldValue,
                NewValue = t.NewValue
            };
    }
}

and that's pretty much it. All dependencies are in one place. Then, when calling a data object from the business logic layer I'd let LINQ do the rest...

var da = new AssetDA();
var entry =  da.GetHistoryRecord(1234);
var domainModelEntry = entry.FromDataObject();

Note that you can define the same to map Domain Model objects to Data Entities.