How to Eagerly Load Related Entities with a Where Clause in Entity Framework?

Efficiently Loading Related Data with Filtering in Entity Framework

This article addresses the challenge of eagerly loading related entities while applying filtering conditions within Entity Framework. The optimal approach depends on your EF version.

Entity Framework 6 Solution

EF6 relies on eager loading and relationship fixup. However, this fixup mechanism doesn't always function correctly with many-to-many relationships. A workaround involves:

1. Disabling Lazy Loading: Prevent lazy loading to ensure all data is retrieved in a single query:

   <code class="language-csharp">Context.Configuration.LazyLoadingEnabled = false;</code>

2. Projection with Filtering: Use a projection to filter related entities during the query:

   <code class="language-csharp">var buses = Context.Busses
                       .Where(b => b.IsDriving)
                       .Select(x => new { b, Passengers = x.Passengers.Where(p => p.Awake) })
                       .AsEnumerable()
                       .Select(x => x.b)
                       .ToList();</code>

3. Automatic Relationship Fixup: EF will automatically populate the Passengers property of each Bus object with only the awake passengers.

4. Data Transfer Objects (DTOs): Use a mapping tool like AutoMapper to convert the retrieved data into DTOs for optimal data transfer.

Entity Framework Core (5.0 and later) Solution

EF Core 5.0 and later versions directly support nested includes with where clauses, simplifying the process:

<code class="language-csharp">var buses = Context.Busses
                    .Include(bus => bus.Passengers.Where(p => p.Awake))
                    .ToList();</code>

Alternative Approach: Third-Party Libraries

Libraries like EntityFramework.DynamicFilters offer a more streamlined method for managing global filters applicable to included collections. This can enhance code readability and maintainability for complex scenarios.

Summary

The best approach for eagerly loading related data with filtering in Entity Framework hinges on the version being used. EF Core 5.0 offers a straightforward solution. For EF6, a workaround involving disabling lazy loading and using projections is necessary. Third-party libraries provide an alternative for improved code organization and flexibility.

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