Add code

Author: stancl

.gitignore

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+vendor/
+composer.lock

composer.json

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+{
+    "name": "stancl/laravel-hasmanywithinverse",
+    "description": "Define HasMany while also setting the inverse relationship in Laravel.",
+    "type": "package",
+    "license": "MIT",
+    "autoload": {
+        "psr-4": {
+            "Stancl\\HasManyWithInverse\\": "src/"
+        }
+    },
+    "authors": [
+        {
+            "name": "Samuel Štancl",
+            "email": "[email protected]"
+        }
+    ],
+    "require": {
+        "illuminate/database": "^6.0|^7.0"
+    }
+}

src/HasManyWithInverse.php

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+<?php
+
+namespace Stancl\HasManyWithInverse;
+
+trait HasManyWithInverse
+{
+    public function hasManyWithInverse($related, $inverse, $foreignKey = null, $localKey = null)
+    {
+        $instance = $this->newRelatedInstance($related);
+
+        $foreignKey = $foreignKey ?: $this->getForeignKey();
+
+        $localKey = $localKey ?: $this->getKeyName();
+
+        return new HasManyWithInverseRelationship($instance->newQuery(), $this, $instance->getTable().'.'.$foreignKey, $localKey, $inverse);
+    }
+}

src/HasManyWithInverseRelationship.php

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+<?php
+
+namespace Stancl\HasManyWithInverse;
+
+use Illuminate\Database\Eloquent\Builder;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\HasMany;
+
+class HasManyWithInverseRelationship extends HasMany
+{
+    protected string $relationToParent;
+
+    public function __construct(Builder $query, Model $parent, $foreignKey, $localKey, $relationToParent)
+    {
+        $this->relationToParent = $relationToParent;
+
+        parent::__construct($query, $parent, $foreignKey, $localKey);
+    }
+
+    public function create(array $attributes = [])
+    {
+        return tap($this->related->newInstance($attributes), function ($instance) {
+            $this->setForeignAttributesForCreate($instance);
+
+            $instance->setRelation($this->relationToParent, $this->getParent());
+
+            $instance->save();
+        });
+    }
+}