Solving the AutoMapper ProjectTo Count Issue in ASP.NET Core 3.1 with Entity Framework

Описание: Discover how to resolve the `AutoMapper ProjectTo ` not working with `Count()` issue in your ASP.NET Core application using Entity Framework Core, ensuring accurate paging and counts.
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Understanding the Issue: AutoMapper ProjectTo and Count()

When working with AutoMapper in an ASP.NET Core application, developers often encounter situations where the ProjectTo<> method does not perform as expected, particularly when used in conjunction with the Count() method. This can lead to discrepancies in data counts, especially when filtering based on related entities.

In this guide, we will explore a common issue involving counting records while projecting them to a view model using AutoMapper. We will demonstrate the problem and provide a clear solution to ensure that paging and counting are in sync.

The Problem

Consider the following scenario where you want to paginate customer records and count the total number of customers, but you want to exclude those without a related status:

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What’s Happening?

Paging Query: The first line correctly creates a SQL query that retrieves a paginated list of customers and includes an INNER JOIN to filter customers by their status.

Count Query: The second line generates a Count() SQL query that ignores the projection and simply counts all records in the Customers table, excluding any filtering based on the related Status.

This discrepancy can lead to incorrect page counts and rendering when a customer has a null StatusId, thereby breaking your application's paging mechanism.

The Solution

To resolve this issue and ensure that the count matches the filtered records, follow these steps:

1. Modify Your Customer Model

Add a nullable foreign-key property to your Customer model. This allows for better handling of relationships and helps generate the correct SQL for both counting and projecting.

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2. Update the Paging Query

Modify the paging query to include a filter that explicitly checks for non-null StatusId values. This ensures that only customers with a related status are included in the result set.

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Now, the SQL produced will look as follows:

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3. Update the Counting Query

Finally, modify the counting query to include the same filter based on StatusId.

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This will generate a simple count query like:

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Conclusion

By following the above steps, you can ensure that your AutoMapper ProjectTo<> method works correctly with counting operations in ASP.NET Core 3.1 using Entity Framework Core. This not only helps maintain accurate paging but also guarantees that all business logic related to filtering on related entities is correctly applied.

For any developers facing similar issues, these practices can significantly simplify and enhance the accuracy of querying with AutoMapper.

If you have further questions or if there's another topic you wish to explore, feel free to comment below!