Angular Material: mat-select default value when using reactive forms

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05-09-2024

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Setting Default Values in Angular Material mat-select with Reactive Forms

Using Angular Material's mat-select component within reactive forms is a common practice for providing user-friendly dropdown menus. However, setting default values for the selection can be tricky, especially when working with arrays of objects. This article will delve into the common challenges and solutions for setting default values in mat-select with reactive forms, drawing insights from Stack Overflow discussions to provide a comprehensive guide.

The Problem:

As illustrated in the Stack Overflow question [link to original SO question], the user faces an issue setting the default value for the owners property of their core object in a reactive form. While the name property updates correctly, the owners array remains empty despite patching the form with the core.owners data.

The Root Cause:

The root cause lies in the way Angular Material's mat-select handles multiple selections. Unlike simple string or number values, it expects the selected values to match the exact objects within the options array. Directly patching the owners array with the core.owners data doesn't work because the mat-select component doesn't recognize these objects as matching the users array provided as options.

The Solution:

The key is to ensure that the core.owners array contains references to the same objects present in the users array. We can achieve this by either:

1. Modifying core.owners: This involves comparing each core.owner with the users array and replacing the core.owner with the matching object from the users array.
2. Modifying users: Alternatively, we can modify the users array to include the core.owners data. This can be done by adding a property like selected to each user object and setting it to true if the user is present in the core.owners array.

Implementation:

Here's an example of modifying the core.owners array to ensure compatibility with the mat-select component:

updateForm() {
  // Find matching users from the users array based on the core.owners array
  const selectedUsers = this.core.owners.map(coreOwner => 
    this.users.find(user => user.id === coreOwner.id)
  );

  this.coreForm.patchValue({
    name: this.core.name,
    owners: selectedUsers // Use the matched users from the users array
  });
}

This approach iterates through the core.owners array, finding the matching user object from the users array based on an appropriate identifier (e.g., id). The resulting array of matched users is then patched into the form, ensuring compatibility with the mat-select component.

Additional Considerations:

• If you have control over the data structures, consider using a shared data model for both core.owners and the users array to avoid this kind of mismatch altogether.
• If your data model is complex and involves multiple arrays and objects, consider using a dedicated service to handle data transformations and synchronization between different data structures. This will make your code cleaner and more maintainable.

Conclusion:

Setting default values in Angular Material's mat-select within reactive forms can present a challenge when working with arrays of objects. By understanding the root cause and implementing a solution that ensures the selected values match the options provided to the mat-select, you can overcome this obstacle and achieve the desired functionality. Remember to consider data model design and data synchronization strategies for a seamless and efficient implementation.