hasOne

class PurchaseDetail extends Model

{

use HasFactory;

public function _items(){

return $this->hasOne(Inventory::class,'id','_item_id')->select('id','_item as _name','_unit_id','_unique_barcode','_pur_rate','_barcode')->with(['_units']);

}

( extends Model – This means PurchaseDetail is an Eloquent model, representing a table ( purchase_details) in the database. )

Relationship Method: _items() method name

$this->hasOne(Inventory::class, 'id', '_item_id')

$this->hasOne(Inventory::class, 'id', '_item_id')

This defines a one-to-one relationship between PurchaseDetail and Inventory.
Parameters:
1. Inventory::class → The related model.
2. id → The column in Inventory (the local key).
3. _item_id → The foreign key in PurchaseDetail.
Meaning:
A PurchaseDetail "has one" Inventory item, where Inventory.id = PurchaseDetail._item_id.

Table	column	Relationship
purchase_details	_item_id	Hasone
Inventory	id	Hasone

->select('id','_item as _name','_unit_id','_unique_barcode','_pur_rate','_barcode')

These columns retrieved from the Inventory table.

So when you access $purchaseDetail->_items, you only get these columns:

id, _name, _unit_id, _unique_barcode, _pur_rate, _barcode

class Inventory extends Model

….
…..

public function _units(){
return $this->hasOne(Units::class,'id','_unit_id')->select('id','_name','_code');

}

Model Name	Table Name	Key Columns	Relationship
PurchaseDetail	purchase_details	id, _item_id, other purchase fields	hasOne → Inventory (_items)
Inventory	inventories	id, _item, _unit_id, _unique_barcode, _pur_rate, _barcode	hasOne → Units (_units)
Units	units	id, _name, _code	-

2. Relationships

PurchaseDetail → Inventory
- A PurchaseDetail belongs to one Inventory item.
- PurchaseDetail._item_id → Inventory.id
- Eloquent relation: hasOne(Inventory::class, 'id', '_item_id')
Inventory → Units
- Each inventory item has one unit.
- Inventory._unit_id → Units.id
- Eloquent relation: hasOne(Units::class, 'id', '_unit_id')