How to Efficiently Convert Non-Collection Child Entities to DTOs in Entity Framework Core?

Efficiently Mapping Non-Collection Child Entities to DTOs in Entity Framework Core

Entity Framework Core simplifies entity-to-DTO (Data Transfer Object) conversion using expression trees. This works well for parent objects and child collections, but handling non-collection child properties requires a different approach.

The Challenge:

Consider this model:

<code class="language-csharp">public class Model
{
    public int ModelId { get; set; }
    public string ModelName { get; set; }
    public virtual ICollection<ChildModel> ChildModels { get; set; }
    public AnotherChildModel AnotherChildModel { get; set; }
}</code>

Directly mapping to a DTO using a simple expression like this fails for AnotherChildModel:

<code class="language-csharp">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 mapping missing
    };</code>

The Solution: Leveraging External Libraries

Several libraries extend expression tree capabilities to handle this scenario:

• LINQKit: Uses the Expandable attribute to mark methods for expansion by the query provider.

<code class="language-csharp">[Expandable(nameof(AsDtoImpl))]
public static ModelDto AsDto(Model model) => AsDtoImpl.Compile()(model);

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

Use .AsExpandable() in your query:

<code class="language-csharp">dbContext.Models
    .Where(m => SomeCriteria)
    .Select(Model.AsDto)
    .AsExpandable()
    .ToList();</code>

• NeinLinq: Similar to LINQKit, using the InjectLambda attribute.

<code class="language-csharp">[InjectLambda]
public static ModelDto AsDto(Model model) { /* ... same implementation as LINQKit ... */ }</code>

Use .ToInjectable() in your query:

<code class="language-csharp">dbContext.Models
    .Where(m => SomeCriteria)
    .Select(Model.AsDto)
    .ToInjectable()
    .ToList();</code>

• DelegateDecompiler: Offers a simpler syntax with the Computed attribute.

<code class="language-csharp">[Computed]
public static ModelDto AsDto(Model model) { /* ... same implementation as LINQKit ... */ }</code>

Use .Decompile() in your query:

<code class="language-csharp">dbContext.Models
    .Where(m => SomeCriteria)
    .Select(Model.AsDto)
    .Decompile()
    .ToList();</code>

These libraries provide efficient solutions for mapping non-collection child entities to DTOs within Entity Framework Core queries, avoiding the limitations of basic expression tree mapping. Choose the library that best suits your project's needs and coding style.

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