The union() Trap: Key-Wise Collisions in Laravel Collections

A common pitfall in PHP development occurs when attempting to combine two collections of scalar values (such as database IDs or UUIDs) using Laravel's union() method. Because the method name evokes mathematical set theory, developers often expect value-based deduplication.

When valid data silently disappears from the resulting collection, the initial impulse is often to treat it as a framework bug. However, this is not a Laravel issue—it is a developer implementation issue.

Laravel's union() method is behaving exactly as designed. Understanding why requires looking at how array keys work in PHP.

Why This Isn't a Bug (It's a Misuse of the Tool)

In PHP, arrays are ordered maps that associate values to keys. When you define a simple list like [46, 53, 821], PHP implicitly assigns numeric keys starting at zero (0 => 46, 1 => 53, 2 => 821).

Laravel’s Collection::union() method delegates directly to PHP’s binary array addition (+) operator under the hood:

public function union($items)
{
    return new static($this->items + $this->getArrayableItems($items));
}

The + operator in PHP performs a key-wise union. It takes all key-value pairs from the left array and appends entries from the right array only if their keys do not already exist in the left array.

Because the method is designed specifically for key preservation, using union() on zero-indexed flat arrays causes positional index collisions. The framework isn't dropping data arbitrarily—your code is instructing PHP to ignore any item whose index position is already occupied.

Concrete Example: Positional Index Collision

Consider merging project IDs from two separate data sources:

// Standard 0-indexed collections:
$timeProjectIds    = collect([46, 53, 821]);  // Keys: 0 => 46,  1 => 53, 2 => 821
$invoiceProjectIds = collect([101, 53, 1126]); // Keys: 0 => 101, 1 => 53, 2 => 1126

$result = $timeProjectIds->union($invoiceProjectIds);

// Resulting Collection: [0 => 46, 1 => 53, 2 => 821]

Because $invoiceProjectIds shares identical positional keys (0, 1, 2), the + operator resolves the key collision by keeping the values from $timeProjectIds and discarding the rest. IDs 101 and 1126 are dropped entirely because their keys collided, not their values.

Guidelines for Laravel Developers

To avoid self-inflicted data loss, apply the right collection operation for your specific data structure:

• Reserve union() Exclusively for Associative Collections Use union() Only when working with key-value pairs (like dictionary maps or custom string keys) where key preservation is required and left-hand key precedence is desired.

• Use concat() + unique() for Flat Lists (Recommended). When combining lists of primitive values (IDs, emails, tags), use concat(). It appends elements while discarding original positional keys. Follow with unique() and values() to deduplicate values and re-index the result:

$uniqueProjectIds = $timeProjectIds
    ->concat($invoiceProjectIds)
    ->unique()
    ->values();

• Use merge() When Re-indexing Collisions is Preferred: Unlike array addition, Collection::merge() appends numeric keys sequentially rather than overwriting key collisions. Pairing merge() with unique() ensures every value across both collections is safely evaluated:

$uniqueProjectIds = $timeProjectIds
    ->merge($invoiceProjectIds)
    ->unique()
    ->values();

Blaming the framework when union() drops items is a mistake. Recognizing that union() is a key-first operation allows you to choose the proper tool—concat() or merge()—for value-first collection merging.