Inverse or Inverted MANY to Many Relationships in Laravel

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Many-to-many relationships are a type of relationship available in relational databases, where multiple records from one table are related to multiple records from another, and usually, at some point, we paginate records in Laravel. For example, if we have a relationship between Posts and Tags, a Post can have many Tags and a Tag can be assigned to multiple Posts.

This means that a record can be associated with many other records. To save this type of relationship, a pivot table is used, which stores at least the primary keys (PKs) of both entities.

In this type of relationship, we can obtain the tags of a post, which would be the direct relationship, or we can obtain the posts associated with a tag, which would be the inverse relationship.

Many-to-many relationships in Laravel are among the most powerful (and also the most confusing) when you start working with Eloquent on real projects. On paper they seem simple, but as soon as you need to access the inverse relationship, filter results, or use more complex pivot tables, the problems begin.

In this guide, I explain how they really work, when to use each type, and which errors to avoid, based on real cases like the use of tags, posts, and courses, not simplified academic examples.

What is a many-to-many relationship in Laravel?

A many-to-many relationship occurs when a record in one table can be related to many records in another table, and vice versa.

Classic examples:

• A post has many tags and a tag belongs to many posts.
• A user has several roles and a role can be assigned to many users.
• A student can be in several courses and a course can have several students.

When to use a many-to-many relationship

Use this relationship when:

• Neither of the two entities truly "owns" the other.
• Both can exist independently.
• You need to query the relationship from both sides.

Difference between one-to-many and many-to-many

In one-to-many, a foreign key lives in a single table.

In many-to-many, you need an intermediate table that connects both.

And this is where the pivot table comes into play.

The pivot table in Laravel

What it is and why it is necessary

The pivot table is the one that stores the relationships between both entities. It does not store "business" information, but rather which record is related to which.

In a real project working with posts and tags, I ended up using a "taggables" table because I needed to reuse tags in different models. That's when I understood that the pivot table is not optional: it is the core of the relationship.

Intermediate table naming convention

Laravel assumes by default:

• Model names in singular
• Alphabetical order
• Example: post_tag

If you don't follow this convention, you will have to specify it explicitly in belongsToMany.

Foreign keys and minimum structure

A basic pivot table has:

• post_id
• tag_id

In more advanced relationships, it may include:

• timestamps
• flags
• types (in polymorphic relationships)

Defining many-to-many relationships with belongsToMany

Working with the direct many-to-many relationship

Before that, let's see how the direct connection is. In this example, I have a normal relationship between Posts and Tags, where a Post has multiple tags and a tag can be assigned to one or many Posts, therefore it is a many-to-many relationship; for this, we have a pivot table called "taggable" in which we store these values: it has the following structure:

• The tag_id column stores the identifiers of the Tag table
• The taggable_id column stores the identifiers of the Post table

Direct many-to-many relationship

If we are going to maintain a relationship in which a tag BELONGS to a Post, we have:

class Post extends Model
{
    protected $fillable = ...
    public function tags()
    {
        return $this->belongsToMany(Tag::class);
        // return $this->morphToMany(Tag::class, 'taggable');  // if you want a polymorphic relationship 
}

Inverse many-to-many relationship

In the inverse model, you must also define belongsToMany, not hasMany.

This is a very common error. If you define it with hasMany, it simply won't work. The problem arises when you want to obtain the relationships inversely, that is, given a tag, you want to obtain all the Posts that belong to that tag; for that, from the tag, we have to create the direct relationship, since with hasMany we CANNOT perform this operation; so:

For tags, which is the inverse relationship (if you want to get the posts of a tag):

class Tag extends Model
{
    protected $fillable = ...
    public function posts()
    {
        return $this->belongsToMany(Post::class);
        // return $this->hasMany(Post::class);
    }
}

Ending up as:

class Tag extends Model
{
    protected $fillable = ['title', 'url_clean'];
    public function post()
    {
        return $this->hasMany(Post::class);
    }
    public function myPosts()
    {
        return $this->belongsToMany(Post::class,'taggables','tag_id','taggable_id');
    }
}

And with this, we can now obtain the direct relationship between Tags and Posts; so:

$posts = $tag::myPost()->get();

Basic example to obtain records

Once both relationships are defined:

$post = Post::find(1);
$tags = $post->tags;

$tag = Tag::find(1);
$posts = $tag->posts;

Accessing data from a many-to-many relationship

Obtaining relationships from the main model

To get the tags of a post:

$posts = Post->with('tags')->get();

Obtaining relationships from the inverse model

Tag::with('posts')->get();

In my case, I have one more relationship, which is for courses, where a post belongs to a course, so:

$courses_tags = Tag::whereHas('myPost', function ($query) {
      return $query->where('type', 'courses');
})->get();

And the relationship or model for our course is:

class Course extends Model
{
    protected $fillable = ['name', 'description', 'image', 'post_id'];
    public function post()
    {
        return $this->belongsTo(Post::class);
    }
}
class Post extends Model
{
    protected $fillable = ['title', 'url_clean', 'content', 'category_id', 'posted', 'description', 'final_content', 'aux_content', 'web_content', 'image', 'path', 'date', 'type'];
    public function category()
    {
        return $this->belongsTo(Category::class);
    }
    public function tags()
    {
        return $this->morphToMany(Tag::class, 'taggable');
    }
}

Loading relationships with eager loading

Whenever you can, use with() to avoid the N+1 problem and improve performance.

Many-to-many between products and tags

Another example of many-to-many relationships: first, an intermediate table named "product_tag" can be created with foreign keys for the product and the tag, i.e., this would be the pivot table. Now, the Laravel Eloquent models and their corresponding relationships can be defined.

In the product model, use the belongsToMany() method to define the relationship with the tag, where a post has multiple tags.

While in the tag model, the same method can be used to define the inverse relationship with products:

class Product extends Model
{
   public function tags()
   {
       return $this->belongsToMany(Tag::class);
   }
}
class Tag extends Model
{
   public function products()
   {
       return $this->belongsToMany(Product::class);
   }
}

To reference them, we have:

$product = Product::find(1);
$tags = $product->tags;

$tag = Tag::find(1);
$products = $tag->products;

Working with the pivot table

Adding extra fields with withPivot

If your pivot table has more columns:

return $this->belongsToMany(Role::class)->withPivot('active');

Handling timestamps in the pivot table

return $this->belongsToMany(Role::class)->withTimestamps();

Accessing pivot data

$role->pivot->active;

Common operations in many-to-many relationships

attach, detach, and sync

$post->tags()->attach(1);
$post->tags()->detach(1);
$post->tags()->sync([1, 2, 3]);

When to use sync and when not

• sync replaces relationships
• attach adds without deleting
• detach removes

Using sync carelessly can delete existing relationship data without you realizing it.

Common errors when synchronizing relationships

• Passing models instead of IDs
• Using sync when you only want to add
• Not validating data before synchronizing

Filtering many-to-many relationships in Laravel

Using whereHas with many-to-many

Tag::whereHas('posts', function ($query) {
   $query->where('type', 'courses');
})->get();

I ended up using this pattern when I needed to get only tags associated with courses, something that isn't usually explained in basic tutorials.

Filtering by pivot fields

return $this->belongsToMany(Role::class)
           ->wherePivot('active', 1);

Real-world advanced filtering cases

• Active tags
• Roles with a valid date
• Status-dependent relationships

Polymorphic many-to-many relationships

Polymorphic relationships are a more advanced topic feared by some, but in essence, it is a mechanism that allows us to share a pivot table and with it its many-to-many relationships (although you can also use it in other types of relationships); definitively, the "problem" we have with the traditional scheme is that the FK or foreign key is specific to a table; therefore, if we want to create a relationship (for example, comments and posts, and then we want to use the comments table also for videos) we must duplicate the relational table (in this example, the comments -video_comments and post_comments-) which implies more work.

But morphic relationships allow us to use the same original table (comments) by adding an additional column (Laravel already does this internally for us) that specifies to whom the relationship belongs (comment).

When to use morphToMany

Use it when:

• An entity (like tags) can be related to different models
• You want to reuse the same pivot table

Difference between belongsToMany and morphToMany

• belongsToMany: classic relationship
• morphToMany: flexible relationship based on type

Real example with tags

class Post extends Model
{
   public function tags()
   {
       return $this->morphToMany(Tag::class, 'taggable');
   }
}

And in the Tag model:

class Tag extends Model
{
   public function posts()
   {
       return $this->morphedByMany(Post::class, 'taggable');
   }
}

Common errors in many-to-many relationships in Laravel

• Using hasMany instead of belongsToMany
  • Classic and very frequent error.
• Foreign key problems
  • Incorrect names
  • Incorrect order
  • Lack of indexes
• Confusion between normal and polymorphic relationships
  • If you don't need polymorphism, don't use it. It complicates things more than it helps.

Best practices for many-to-many relationships in Laravel

Database design

• Clear names
• Well-defined keys
• Indexes on pivot tables

Performance and efficient queries

• Use eager loading
• Filter in the database, not in PHP

When to rethink the model

• If the pivot table starts to have too much logic, it might need to be its own model.

Frequently asked questions about many-to-many relationships in Laravel

• What happens if I don't use a pivot table?
  • You cannot correctly model a many-to-many relationship.
• Can I use many-to-many without an intermediate model?
  • Yes, but when the pivot table grows, it's better to create its own model.
• How to debug a many-to-many relationship that isn't working?
  • Check table names
  • Check foreign keys
  • Test the query directly in SQL

Conclusion

Mastering many-to-many relationships in Laravel makes a big difference between knowing how to use Eloquent and using Eloquent well. When you understand how the pivot table works, how to define the inverse relationship, and how to filter correctly, you start solving real problems without patches.

Learn not only how to handle many records but also how to perform operations in a single unit of work through database transactions.