How Can I Reuse Code for Mapping Single Child Properties to DTOs in EF Core?

Streamlining DTO Mapping for Single Child Properties in EF Core

Entity Framework Core (EF Core) simplifies object-to-DTO (Data Transfer Object) conversion using expression trees. This works well for child collections, but mapping single child properties can be less intuitive. This article explores solutions for reusing code and efficiently handling this scenario.

The Challenge: Single Child Property Mapping

Directly mapping a single child property to a DTO often requires manual expression additions, leading to repetitive code. Consider this example:

public static Expression<Func<Model, ModelDto>> AsDto =>
    model => new ModelDto
    {
        ModelId = model.ModelId,
        ModelName = model.ModelName,
        ChildModels = model.ChildModels.AsQueryable().Select(ChildModel.AsDto).ToList(),
        AnotherChildModel = new AnotherChildModelDto // Manual mapping here
        {
            AnotherChildModelId = model.AnotherChildModel.AnotherChildModelId
        }
    };

The Solution: Leveraging External Libraries

Several libraries provide elegant solutions by effectively adding a .Select()-like functionality for single entities, thus enabling code reuse:

1. LINQKit:

LINQKit allows for the expansion of expressions, handling the single-child mapping dynamically.

[Expandable(nameof(AsDtoImpl))]
public static ModelDto AsDto(Model model)
{
    _asDtoImpl ??= AsDtoImpl().Compile();
    return _asDtoImpl(model);
}

private static Func<Model, ModelDto> _asDtoImpl;

private static Expression<Func<Model, ModelDto>> AsDtoImpl =>
    model => new ModelDto
    {
        ModelId = model.ModelId,
        ModelName = model.ModelName,
        ChildModels = model.ChildModels.AsQueryable().Select(ChildModel.AsDto).ToList(),
        AnotherChildModel = model.AnotherChildModel.AsDto() // LINQKit handles this
    };

2. NeinLinq:

NeinLinq offers a similar approach using lambda injection.

[InjectLambda]
public static ModelDto AsDto(Model model)
{
    _asDto ??= AsDto().Compile();
    return _asDto(model);
}

private static Func<Model, ModelDto> _asDto;

private static Expression<Func<Model, ModelDto>> AsDto =>
    model => new ModelDto
    {
        ModelId = model.ModelId,
        ModelName = model.ModelName,
        ChildModels = model.ChildModels.AsQueryable().Select(ChildModel.AsDto).ToList(),
        AnotherChildModel = model.AnotherChildModel.AsDto() // NeinLinq handles this
    };

3. DelegateDecompiler:

DelegateDecompiler offers a more concise syntax using computed properties. Note that this approach might not always be suitable depending on the complexity of your mapping.

[Computed]
public static ModelDto AsDto(Model model)
=> new ModelDto
{
    ModelId = model.ModelId,
    ModelName = model.ModelName,
    ChildModels = model.ChildModels.AsQueryable().Select(ChildModel.AsDto).ToList(),
    AnotherChildModel = model.AnotherChildModel.AsDto() // DelegateDecompiler handles this
};

Each library requires appropriate setup and configuration to function correctly within your project.

Conclusion:

These libraries offer efficient solutions for reusing code when mapping single child properties to DTOs in EF Core, reducing redundancy and improving maintainability. Choosing the right library depends on project preferences and complexity. The core benefit is a consistent and DRY (Don't Repeat Yourself) approach to DTO mapping.

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