Fix the Failure to Update Article Tags in a Laravel 8 Blog Application (PHP)

I’ve been working on a blogging application (PHP) in Laravel 8 and hit a puzzling issue while updating article tags. In my app, articles and tags have a many-to-many relationship, with an intermediate pivot table (article_tag) to manage their connections. The idea was simple: let users assign multiple tags to an article and update them when needed. However, my update method started throwing an integrity constraint violation error:

Integrity constraint violation: 1062 Duplicate entry ‘306-1’ for key ‘PRIMARY’

After hours of debugging, I finally realized what was going wrong, and I want to share my experience, the relevant code, and what I learned from this process.

The Solution

In my Tag model, I allowed mass assignment for the tag’s name and defined the many-to-many relationship to articles:

class Tag extends Model
{
    use HasFactory;

    protected $fillable = ['name'];

    public function articles()
    {
        return $this->belongsToMany(Article::class);
    } 
}

For the Article model, I added a tags() method to define the inverse relationship:

public function tags()
{
    return $this->belongsToMany(Tag::class)->as('tags');
}

The Controller Methods

I set up two methods in my ArticleController for editing and updating an article.

Editing the Article

The edit() method fetches the article, the list of all tags, and the currently selected tags. This is used to pre-populate the form:

public function edit($id)
{
    $article = Article::find($id);
    return view('dashboard/edit-article', [
        'categories' => $this->categories(),
        'tags' => $this->tags(),
        'selected_tags' => $this->tags()->pluck('id')->toArray(),
        'article' => $article
    ]);
}

Updating the Article

The update() method validates the request, processes an optional image upload, updates article fields, and then attaches tags:

public function update(Request $request, $id)
{
    $validator = Validator::make($request->all(), $this->rules, $this->messages);

    if ($validator->fails()) {
        return redirect()->back()->withErrors($validator->errors())->withInput();
    }

    $fields = $validator->validated();
    $article = Article::find($id);

    // Handle image upload
    if (isset($request->image)) {
        $imageName = md5(time()) . Auth::user()->id . '.' . $request->image->extension();
        $request->image->move(public_path('images/articles'), $imageName);
    } else {
        $imageName = $article->image;
    }

    $article->title = $request->get('title');
    $article->short_description = $request->get('short_description');
    $article->category_id = $request->get('category_id');
    $article->tags[] = $request->get('tags[]'); // <== This line is problematic
    $article->featured = $request->has('featured');
    $article->image = $request->get('image') == 'default.jpg' ? 'default.jpg' : $imageName;
    $article->content = $request->get('content');

    // Save changes to the article
    $article->save();

    // Attach tags to article
    if ($request->has('tags')) {
        $article->tags()->attach($request->tags);
    }

    return redirect()->route('dashboard.articles')
        ->with('success', 'The article titled "' . $article->title . '" was updated');
}

The Form

The form for editing an article uses a multi-select box for tags:

<select name="tags[]" id="tags" class="form-control" multiple="multiple">
    @foreach ($tags as $tag)
        <option value="{{ $tag->id }}" {{ in_array($tag->id, $selected_tags) ? 'selected' : '' }}>
            {{ $tag->name }}
        </option>
    @endforeach
</select>

What Went Wrong

When I attempted to update an article, Laravel was trying to insert duplicate entries in the article_tag pivot table. The error message indicated a duplicate primary key (like ‘306-1’), meaning the combination of article ID and tag ID already existed. After some thought and debugging, I discovered that the issue was caused by the following line in my update method:

$article->tags[] = $request->get('tags[]');

This line attempts to directly modify the tags attribute on the Article model. This doesn’t work with many-to-many relationships and causes unintended behavior when combined with the subsequent attach() call. Instead of modifying the model’s relationship property, the proper approach is to use Eloquent’s relationship syncing methods.

The Fix

I removed the problematic line and replaced the attach logic with a sync. Using sync() instead of attach() ensures that the pivot table is updated correctly without duplicates—it removes any tags not in the incoming list and attaches the new ones.

Here’s the corrected update method:

public function update(Request $request, $id)
{
    $validator = Validator::make($request->all(), $this->rules, $this->messages);

    if ($validator->fails()) {
        return redirect()->back()->withErrors($validator->errors())->withInput();
    }

    $fields = $validator->validated();
    $article = Article::find($id);

    // Handle image upload
    if (isset($request->image)) {
        $imageName = md5(time()) . Auth::user()->id . '.' . $request->image->extension();
        $request->image->move(public_path('images/articles'), $imageName);
    } else {
        $imageName = $article->image;
    }

    $article->title = $request->get('title');
    $article->short_description = $request->get('short_description');
    $article->category_id = $request->get('category_id');
    // Removed the problematic line that tried to assign tags directly.
    $article->featured = $request->has('featured');
    $article->image = $request->get('image') == 'default.jpg' ? 'default.jpg' : $imageName;
    $article->content = $request->get('content');

    // Save changes to the article
    $article->save();

    // Update pivot table using sync() to avoid duplicate entries.
    if ($request->has('tags')) {
        $article->tags()->sync($request->tags);
    }

    return redirect()->route('dashboard.articles')
        ->with('success', 'The article titled "' . $article->title . '" was updated');
}

Using sync() has a couple of advantages:

• Prevents Duplicates: It ensures that only the selected tag IDs remain in the pivot table.
• Simpler Maintenance: It automatically detaches any tags not present in the request, keeping the relationship in sync with the form input.

Final Thoughts

This experience taught me a valuable lesson about working with many-to-many relationships in Laravel. Instead of trying to manually adjust the relationship by pushing values into an array property, it’s much more reliable to use Eloquent’s built-in methods like sync(), which handle the complexities of pivot tables behind the scenes.

I love these kinds of challenges because they push me to dive deeper into Laravel’s powerful ORM and learn better practices. If you ever run into a similar issue, remember to review how you’re updating relationships. Often, the solution is already provided by Eloquent—just use the right method!