get many to many values and which of them are selected in laravel

Contacts:
id
name

Tags:
id
name

ContactTags:
contact_id
tag_id

In Contacts model:

public function tags()
{
    return $this->belongsToMany(Tags::class, "contacts_tags", "contact_id", "tag_id");
}

So if I do

$contact = Contacts::findOrFail($id);
dd($contact->tags);

I successfully get the tags associated with the contact. But how can I get all tags and a flag indicating which one of those is associated?

I'm trying to prevent fetching all tags, loop them and with each iteration loop all contact_tags and check if tag_id matches. I want to display a list of checkboxes with all tags and check the ones that are in the relation.

This code can help you, but I'm using the SELECT multiple component. You can easily adapt it to use the CHECKBOX component.

Contacts model:

public function tags()
{
    return $this->belongsToMany(Tags::class, "contacts_tags", "contact_id", "tag_id");
}

ContactController.php

public function edit(Contact $contact)
{
    $tags = Tag::all();
    return view('contacts.edit',compact('contact', 'tags'));        
}

edit.blade.php

<div class="row">
        <div class="col">
            <div class="form-group">
                <strong>Tags:</strong>
                <select name="tags_id[]" multiple>
                    @foreach ($tags as $tag)
                        @if( $contact->tags->contains($tag) )
                            <option value="{{ $tag->id }}" selected>{{ $tag->name }}</option>
                        @else
                            <option value="{{ $tag->id }}">{{ $tag->name }}</option>
                        @endif
                    @endforeach
                </select>
            </div>
        </div>
    </div>

Update in ContactController.php

public function update(Request $request, Post $contact)
{   
    $validatedData = $request->validate([
            'tags_id' => ['array'],
        ]);

    $contact->update($request->all());
    $contact->tags()->sync($validatedData['tags_id']); 

    return redirect()->route('contact.index')->with('success', 'Contact successfully updated!');
}

The validation is just an example. The $validatedData has no use here, but it can be used to update the contact if you validate the other fields.