Laravel Relationship from array in database

you can't get condition with relation you need to make it like this

public function getConditionsAttribute($conditions)
    {
        return App\Condition::whereIn('id',collect($conditions)->pluck('condition_id'))->get();
    }

and it will dynamically get with your data, and this will make it like with logic

Alerts Model

Schema::create('alerts', function (Blueprint $table) {
            $table->bigIncrements('id');
            $table->integer('user_id');
            $table->float('price_low')->nullable();
            $table->float('price_high')->nullable();
            $table->unsignedBigInteger('conditions')->nullable();
            $table->foreign('conditions')->references('id')->on('conditions');
            $table->softDeletes();
            $table->timestamps();
        });

Conditions model

Schema::create('conditions', function (Blueprint $table) {
            $table->bigIncrements('id');
            $table->string('condition_value')->nullable();
            $table->softDeletes();
            $table->timestamps();
        });

insert data in Alerts table multiple records for a user_id ig a user id having multiple conditions such as:

user_id: 1 condition: 1

user_id: 1 condition: 2

user_id: 1 condition: 3

Why aren't you modeling your relationship as a many to many between alerts and conditions?

I would use the following migrations:

Schema::create('alerts', function (Blueprint $table) {
    $table->bigIncrements('id');
    $table->unsignedBigInteger('user_id'); // I assume your users table uses a bigIncrements for id
    $table->decimal('price_low')->nullable(); // Do not use FLOAT type for prices as is can cause rounding issues.
    $table->decimal('price_high')->nullable();
    $table->softDeletes();
    $table->timestamps();

    $table->foreign('user_id')->references('id')->on('users');
});

Schema::create('alert_condition', function (Blueprint $table) {
    $table->bigIncrements('id');
    $table->unsignedBigInteger('alert_id');
    $table->unsignedBigInteger('condition_id');
    $table->timestamps();

    $table->foreign('alert_id')->references('id')->on('alerts');
    $table->foreign('condition_id')->references('id')->on('conditions');
});

Schema::create('conditions', function (Blueprint $table) {
    $table->bigIncrements('id');
    $table->string('value');
    $table->timestamps();
});

To insert the conditions you now have to use:

DB::table('conditions')->insert([
    ['value' => 'New'],
    ['value' => 'Second Hand'],
    ['value' => 'Ex Demo'],
]);

The only additional thing that you need to be aware is that each association between an alert and a condition will be a record in the alert_condition pivot table with the foreign keys of the two models that you want to connect.

This can be done in a number of ways. To stick with your previous conditions inserts you can use:

DB::table('alert_condition')->insert([
    ['alert_id' => 1, 'condition_id' => 1],
    ['alert_id' => 1, 'condition_id' => 2],
    ['alert_id' => 2, 'condition_id' => 1],
    // ...
]);

I would then use the following models:

Alerts Model

<?php

namespace App;

use Illuminate\Database\Eloquent\Relations\Pivot;
use Illuminate\Database\Eloquent\SoftDeletes;

class Alerts extends Pivot
{
    use SoftDeletes;

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

    public function conditions()
    {
        return $this->belongsToMany(Condition::class);
    }
}

Condition Model

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Condition extends Model
{
    public function alerts()
    {
        return $this->belongsToMany(Alert::class);
    }
}

Then in your controller or wherever you need you can query like this:

// Assuming your authentication works with the User model and that model
// has a correct hasMany alerts() relationship.
$user = Auth::user();

// This would print out all the alerts for the current logged in user
// and each alert will contain the conditions values associated to that alert.
dump($user->alerts()->with('conditions')->get());

// If you also need to keep the user with its data, just eager load the relations
dump($user->load('alerts.conditions'));