Скачать или смотреть Resolving FormatException Issues in NHibernate: A Guide to Using Detached Queries and Subqueries

Learn how to resolve `FormatException` issues in NHibernate when working with detached queries and subqueries. This guide provides a clear approach to rectify common mistakes in your code.
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Understanding the FormatException in NHibernate Criteria Queries

If you're delving into NHibernate Criteria Queries, you may encounter a frustrating FormatException when implementing detached queries and subqueries. This error typically indicates that the string format being used for comparison is incorrect. In this guide, we’ll explore a common scenario that leads to this exception, share the necessary code adjustments to prevent it, and explain why these changes work.

The Problem

As you learn to use NHibernate Criteria Queries, you might set up models and queries only to run into errors. Consider the following case, where the Cat model features attributes like Id, Name, Sex, Weight, and a CatStore. If you attempt to execute a query that compares the Weight of each Cat to the average weight calculated through detached criteria, you might come across this exception:

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This often happens when NHibernate tries to convert a string parameter for comparison, which is not handled correctly.

The Cause of the Error

Looking closely at the provided query:

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The subsequent line:

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is where the FormatException is encountered. The method Subqueries.Gt() is meant for comparing a fixed value (like 11.5) against the result of a subquery. When passed "Weight" as a string, NHibernate struggles to convert it to the appropriate data type for comparison, leading to format issues.

The Solution

The solution is straightforward: instead of using Subqueries.Gt(), you should use Subqueries.PropertyGt(), which is designed for comparing property values directly. Here's how you can adjust your code:

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Breakdown of the Fix

Identify the Method: Replace Subqueries.Gt() with Subqueries.PropertyGt().

Parameter Type Handling: The PropertyGt() method takes the property name as a string ("Weight") and the result of the detached subquery (avgWeight), ensuring NHibernate can correctly interpret the types during the comparison.

Correct Execution: This adjustment allows NHibernate to handle your query effectively and prevents format mismatches.

Conclusion

Encountering errors, such as FormatException, can be a common hurdle when working with NHibernate. However, understanding the underlying cause and making slight adjustments can help you navigate these issues confidently. As you continue to explore NHibernate Criteria Queries, keep this solution in mind to avoid unnecessary frustrations!

By embracing best practices in querying and understanding how NHibernate processes data types, you'll enhance your coding experience and improve the efficiency of your applications.