How to Flatten Nested Entities Using LINQ: The Opposite of SelectMany Explained

Описание: Learn how to flatten related entities in Entity Framework Core using LINQ by projecting navigation properties. Replace complex joins with simple, efficient LINQ queries to retrieve parent entity data alongside specific related child entities.
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This video is based on the question https://stackoverflow.com/q/79366230/ asked by the user 'Zachary Scott' ( https://stackoverflow.com/u/84424/ ) and on the answer https://stackoverflow.com/a/79366412/ provided by the user 'Steve Py' ( https://stackoverflow.com/u/423497/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Introduction

When working with related tables in a database—such as Product and its multiple ProductImage records—you might want to flatten nested records into a single result per parent entity. This process is effectively the opposite of LINQ’s SelectMany, which flattens collections into multiple rows.

A typical SQL solution might involve multiple joins to pick specific related records as columns, but this can lead to complex and sometimes inefficient queries.

The Problem

You want to flatten the first three images of a product into one record with columns like ImagePath1, ImagePath2, and ImagePath3. Your SQL might look like this:

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Trying to convert this SQL directly in LINQ often leads to queries that use subqueries per column rather than joins, which looks less familiar but can actually be more efficient.

Modern LINQ Approach Using Navigation Properties

Instead of manually joining tables in LINQ, leverage EF Core's navigation properties to access related data. This approach is clearer, maintainable, and lets EF Core generate efficient SQL.

1. Ensure Navigation Setup

Your Product entity should define a collection of related ProductImage entities:

[[See Video to Reveal this Text or Code Snippet]]

2. Project Specific Images in Your Query

Use LINQ projections to select the desired images by filtering on ImageId:

[[See Video to Reveal this Text or Code Snippet]]

EF Core translates this into SQL with correlated subqueries, which is often more efficient and simpler than multiple self-joins.

3. Handling Nulls

In LINQ to Objects, you’d normally use the null-conditional operator to avoid exceptions:

[[See Video to Reveal this Text or Code Snippet]]

However, EF Core can translate the query properly without explicit null checks, so stick with direct property access to avoid translation issues.

4. Alternative: Return a List of Images

If you want a cleaner structure or variable number of images:

[[See Video to Reveal this Text or Code Snippet]]

This provides a list of up to three image paths, which you can iterate over in the application.

Summary

Use navigation properties instead of explicit joins in LINQ.

Project the related data using FirstOrDefault or collection Take methods.

EF Core efficiently translates these projections into optimized SQL with correlated subqueries.

This approach is cleaner, more maintainable, and often more efficient than manually writing multiple joins.

By embracing EF Core’s relational features, you can flatten nested entities easily and write expressive, performant LINQ queries.