One-to-one (hasOne, belongsTo) in Laravel - Step-by-Step Execution

Concept Flow - One-to-one (hasOne, belongsTo)

Define Model A

↓

Add hasOne relation in Model A

↓

Define Model B

↓

Add belongsTo relation in Model B

↓

Query Model A -> get related Model B

↓

Query Model B -> get related Model A

This flow shows how to set up and use one-to-one relations between two models using hasOne and belongsTo in Laravel.

Execution Sample

Laravel

class User extends Model {
  public function phone() {
    return $this->hasOne(Phone::class);
  }
}

class Phone extends Model {
  public function user() {
    return $this->belongsTo(User::class);
  }
}

Defines a one-to-one relation where a User has one Phone, and a Phone belongs to a User.

Execution Table

Step	Action	Model	Relation Method Called	Query Generated	Result
1	Call user->phone()	User	hasOne	SELECT * FROM phones WHERE user_id = ?	Phone model instance or null
2	Call phone->user()	Phone	belongsTo	SELECT * FROM users WHERE id = ?	User model instance or null
3	Access phone property on user	User	property access	Uses cached relation	Phone instance returned
4	Access user property on phone	Phone	property access	Uses cached relation	User instance returned
5	No related record found	User or Phone	relation call	Query returns empty	null returned

💡 Queries stop after fetching related model or returning null if none found

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
user	User instance	User instance	User instance	User instance with phone loaded	User instance with phone loaded
phone	null	Phone instance or null	Phone instance or null	Phone instance	Phone instance
phone->user	null	null	User instance or null	User instance	User instance

Key Moments - 3 Insights

Why does user->phone() return null sometimes?

Because the related phone record may not exist in the phones table for that user_id, as shown in execution_table step 5.

Why do we use hasOne in User and belongsTo in Phone?

hasOne defines the owning side with foreign key in the other table; belongsTo defines the inverse side with foreign key in this model, as shown in execution_sample.

Does accessing user->phone cache the result?

Yes, after the first query, Laravel caches the related model, so repeated access does not run new queries, as shown in execution_table step 3.

Visual Quiz - 3 Questions

Look at the execution_table, what query runs when calling phone->user()?

A SELECT * FROM phones WHERE user_id = ?

B SELECT * FROM users WHERE id = ?

C SELECT * FROM phones

D SELECT * FROM users

💡 Hint

Check execution_table row 2 under 'Query Generated'

At which step does Laravel return null if no related record exists?

A Step 5

B Step 1

C Step 3

D Step 4

💡 Hint

Look at execution_table row 5 'Result' column

If you remove belongsTo from Phone, what happens when calling phone->user()?

A It returns the User instance anyway

B It runs the hasOne query instead

C It throws an error or returns null

D It caches the phone relation

💡 Hint

belongsTo defines the inverse relation needed for phone->user(), see execution_sample

Concept Snapshot

One-to-one relations link two models where one owns the other.
Use hasOne in the owning model to define the relation.
Use belongsTo in the owned model to define the inverse.
Queries fetch related model by foreign key.
Accessing relation caches the result for reuse.

Full Transcript

In Laravel, one-to-one relations connect two models so each instance of one model relates to exactly one instance of another. We define hasOne in the model that owns the other, meaning it expects the foreign key in the related model's table. The related model uses belongsTo to point back. When you call the relation method, Laravel runs a query to find the related record by foreign key. If found, it returns the related model instance; if not, it returns null. Accessing the relation caches the result to avoid repeated queries. This setup helps keep data connected simply and clearly.