Laravel Collections

Quick Decision Guide

"Which method do I use?"

Find items:

Need	Method
First item matching condition	first(fn) or firstWhere('key', 'value')
First item or throw	firstOrFail(fn)
Exactly one match or throw	sole(fn)
Check value exists	contains('value') or contains('key', 'value')
Check key exists	has('key')
All items match condition	every(fn)

Transform items:

Need	Method
Transform each item	map(fn)
Transform + flatten one level	flatMap(fn)
Extract one field	pluck('name')
Extract field, keyed by another	pluck('name', 'id')
Select multiple fields	select(['id', 'name'])
Change keys	mapWithKeys(fn) or keyBy('id')
Map into class instances	mapInto(DTO::class)

Filter items:

Need	Method
Keep matching items	filter(fn)
Remove matching items	reject(fn)
Filter by field value	where('status', 'active')
Filter by field in list	whereIn('status', ['active', 'pending'])
Keep specific keys only	only(['id', 'name'])
Remove specific keys	except(['password'])
Remove duplicates	unique('email')

Group & organize:

Need	Method
Group by field	groupBy('category')
Group with custom key+value	mapToGroups(fn)
Split pass/fail	partition(fn)
Key by field (1 item per key)	keyBy('id')
Split into chunks	chunk(100)

Aggregate:

Need	Method
Sum values	sum('price')
Average	avg('score')
Min/Max	min('price') / max('price')
Reduce to single value	reduce(fn, $initial)
Count occurrences	countBy('type')
Percentage matching	percentage(fn)

Combine collections:

Need	Method
Merge (overwrites string keys)	merge($other)
Append values (re-indexes)	concat($other)
Keep only shared values	intersect($other)
Get values not in other	diff($other)
Pair items by index	zip($other)

Mutability Rules

Immutable (return new collection) - almost everything: map, filter, reject, sort, merge, pluck, unique, ...

Mutating (modify in place, return $this) - memorize these: transform, push, prepend, put, pull, pop, shift, splice, forget

// WRONG mental model:
$filtered = $collection->push('item'); // $collection IS modified

// Correct approach when you want immutability:
$new = $collection->concat(['item']); // $collection is unchanged

Common Pitfalls

1. each() vs map() vs transform()

// each(): Side-effects, returns ORIGINAL collection
$users->each(fn($u) => $u->notify(new Welcome));

// map(): Transformation, returns NEW collection (original unchanged)
$names = $users->map(fn($u) => $u->name);

// transform(): Transformation, MUTATES original collection
$users->transform(fn($u) => $u->name); // $users is now a collection of names!

2. pluck() key overwrites

When using pluck('name', 'id'), duplicate keys silently overwrite:

collect([
    ['id' => 1, 'name' => 'Alice'],
    ['id' => 1, 'name' => 'Bob'],   // Overwrites Alice
])->pluck('name', 'id');
// [1 => 'Bob']  -- Alice is lost!

Use groupBy if duplicates are expected.

3. filter() without callback removes all falsy values

collect([0, 1, '', 'hello', null, false, []])->filter()->all();
// [1, 'hello'] -- 0, '', null, false, [] are ALL removed

Use whereNotNull() if you only want to remove nulls.

4. N+1 queries with collection iteration

// BAD: N+1 queries
$users->each(fn($user) => $user->posts->count());

// GOOD: Eager load first
$users->load('posts');
$users->each(fn($user) => $user->posts->count());

5. Memory with large datasets

// BAD: Loads all users into memory
User::all()->each(fn($u) => process($u));

// GOOD: Constant memory with cursor
User::cursor()->each(fn($u) => process($u));

// GOOD: Chunked processing
User::chunk(1000, fn($users) => $users->each(fn($u) => process($u)));

6. sort() preserves keys

collect([3, 1, 2])->sort()->all();
// [1 => 1, 2 => 2, 0 => 3] -- keys preserved!

collect([3, 1, 2])->sort()->values()->all();
// [1, 2, 3] -- use values() to re-index

Higher-Order Messages

Proxy pattern for clean, concise collection operations:

// Instead of:
$users->map(function ($user) { return $user->name; });

// Write:
$users->map->name;

// Works with methods too:
$users->each->markAsVip();
$users->sum->votes;
$users->filter->isAdmin();
$users->sortBy->name;
$users->reject->isBlocked();
$users->groupBy->department;

Supported on: average, avg, contains, each, every, filter, first, flatMap, groupBy, keyBy, map, max, min, partition, reject, skipUntil, skipWhile, some, sortBy, sortByDesc, sum, takeUntil, takeWhile, unique.

Extending with Macros

Register custom collection methods (typically in a service provider's boot method):

use Illuminate\Support\Collection;

Collection::macro('toUpper', function () {
    return $this->map(fn($value) => strtoupper($value));
});

// Usage
collect(['foo', 'bar'])->toUpper(); // ['FOO', 'BAR']

Reference Files

• Full method reference: See references/methods.md for every Collection method with signatures and descriptions
• LazyCollection: See references/lazy-collections.md for LazyCollection-specific methods, usage patterns, and when to choose lazy vs eager