Implementing a Custom Validator in Angular for Minimum Selected Items in Nested Arrays

Описание: Learn how to create an effective custom validator in Angular to ensure that at least one item is selected from nested form arrays.
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This video is based on the question https://stackoverflow.com/q/75212991/ asked by the user 'Andrew Howard' ( https://stackoverflow.com/u/1402073/ ) and on the answer https://stackoverflow.com/a/75217858/ provided by the user 'Owen Kelvin' ( https://stackoverflow.com/u/13680115/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Implementing a Custom Validator in Angular for Minimum Selected Items in Nested Arrays

When working with forms in Angular, you may often need to set custom validation rules to enforce certain conditions. One common challenge is to ensure that the user selects at least one item from a series of nested arrays. In this guide, we will delve into a practical example of implementing a custom validator to accomplish this.

The Problem Statement

Imagine you have a form that contains several nested arrays: universities, cities, and countries. Your goal is to verify that at least one of these arrays contains at least one item. This requirement ensures that users don't skip essential selections, helping improve data integrity.

Here is a simplified version of your form setup in Angular:

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

Understanding the Current Validator Code

You have attempted to create a custom validator function to check the number of items in these nested arrays, but it seems like there is a failure in correctly calculating the total selected items.

Here is the code you provided:

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

The Issue

The main problem lies in the line:

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

This line doesn't actually update the accumulated value (acc). Instead, it just performs an addition and does nothing with it, which results in the totalSelected always being zero.

Solution: Updating the Validator

To fix the issue, you need to assign the sum back to acc. Here’s how to correctly adjust the code:

Enhanced Validator Code

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

Key Changes Made

Accumulate Correctly: The line acc + = value.value.length; properly updates the accumulator.

Return the Value: The return acc; statement ensures that the updated value is carried over to the next iteration of the reduction.

Validation Logic: The final condition checks if totalSelected meets the specified minimum.

Conclusion

Creating a custom validator for nested arrays in Angular forms is straightforward once you understand how to traverse and aggregate values properly. By ensuring that the total count of selected items is accurately calculated, you can enforce any required rules effectively.

Feel free to explore additional conditions and expand your validation requirements as needed. Happy coding!