How to Select Many with Multiple Many-to-Many Relationships in EFCore

A comprehensive guide on performing complex `many-to-many` relationships queries in Entity Framework Core, particularly focusing on aggregation and grouping.
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Understanding Many-to-Many Relationships in EFCore

When working with complex data models in Entity Framework Core (EFCore), you may encounter scenarios involving multiple many-to-many relationships. This situation can make querying and aggregating data tricky. In this post, we will look at how to effectively query models that have multiple many-to-many relationships, using a practical example.

The Problem

Imagine you have three models: A, B, and C. Here is how the relationships work:

Model A has a many-to-many relationship with model B.

Model A also has a many-to-many relationship with model C.

Your goal is to create an aggregation that provides the following results for each instance of model A:

a_id – the identifier of A

b_value_sum – the sum of certain values from B associated with A

c_value_sum – the sum of certain values from C associated with A

c_count – the count of related C entities

The output table should look like this:

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

Initial Attempts

Your initial tries to solve this problem using Entity Framework included various LINQ queries. Here’s one attempt:

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

This approach didn't work as expected because it didn't allow you to select any values from B or C.

Another approach you tried was:

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

However, this also failed as z was of type B and not A.

The Solution: Using Query Syntax

To achieve the desired result, it’s simpler and more maintainable to use Query Syntax in C# . Below is the solution that works for your scenario:

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

Breaking Down the Solution

From Clauses:

The first from selects all instances of A.

The second from selects related instances of B for each A.

The third from selects C instances, using DefaultIfEmpty() to perform a left join. This ensures you also get instances of A that do not have related C entries.

Group By:

After selecting the required models, group them by a.Id. This allows us to aggregate values for each unique instance of A.

Select New:

Finally, select a new anonymous object, including the sums of values from B and C, as well as the count of related C entities.

Conclusion

By leveraging query syntax in EFCore, we can create clear, organized, and more maintainable queries for complex scenarios involving many-to-many relationships. This method can handle aggregations effectively, ensuring you retrieve all the necessary data you need.

With this approach, you can now easily generate the desired output, helping you to make data-driven decisions based on your aggregated data.

If you found this guide helpful, feel free to share your thoughts and experiences with many-to-many relationships in EFCore in the comments below!

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