How to relate three tables in laravel

Question

I am trying to develop a p2p app in laravel.

I have three tables in my database namely users, loans, and installments.

Now, users and loans can have multiple installments and each installment will belong to a user and a loan.

So how do I define this relationship? Do I need a polymorphic relationship? If so, how do relate a user and a loan to a single installment?

Answer 1

each installment will belong to a user and a loan.

That pretty much answers the question.

If your requirement was "each installment will belong to either an user or a loan", then the relationship would be polymorphic.

Minimal table structure

Schema::create('users', function (Blueprint $table) {
    $table->bigIncrements('id');
    ... // other users fields or indexes.
});

Schema::create('loans', function (Blueprint $table) {
    $table->bigIncrements('id');
    ... // other loans fields or indexes.
});

Schema::create('installments', function (Blueprint $table) {
    $table->bigIncrements('id');
    $table->foreignId('user_id')->constrained('users');
    $table->foreignId('loan_id')->constrained('loans');
    ... // other installments fields or indexes.
});

IF an Installment's user_id or loan_id can be null, add ->nullable() before ->constrained(). For example:

Schema::create('installments', function (Blueprint $table) {
    ...
    $table->foreignId('user_id')->nullable()->constrained('users');
    ...
});

IF you can't have more than one installment associated with the same user/loan pair, add a unique index.

Schema::create('installments', function (Blueprint $table) {
    ...
    $table->unique(['user_id', 'loan_id']);
    ...
});

Relationships to define

User model:

• public function installments() { return $this->hasMany(Installment::class); }
• public function loans() { return $this->belongsToMany(Loan::class, 'installments'); }

Loan model:

• public function installments() { return $this->hasMany(Installment::class); }
• public function users() { return $this->belongsToMany(User::class, 'installments'); }

Installment model:

• public function loan() { return $this->belongsTo(Loan::class); }
• public function user() { return $this->belongsTo(User::class); }

https://laravel.com/docs/eloquent-relationships#inserting-and-updating-related-models

https://laravel.com/docs/eloquent-relationships#syncing-associations

https://laravel.com/docs/eloquent-relationships#updating-a-record-on-the-intermediate-table

syncWithoutDetaching() and updateExistingPivot() are particularly important methods if you have a unique constraint.