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Table of Contents
Problem description
Analysis of the cause of the problem
Solution
Notes and best practices
Summarize
Home Backend Development PHP Tutorial Solve the problem that the associated data is empty in the Eloquent relationship: Ensure that the foreign key is selected

Solve the problem that the associated data is empty in the Eloquent relationship: Ensure that the foreign key is selected

Jul 23, 2025 pm 06:48 PM
laravel

Solve the problem that the associated data is empty in the Eloquent relationship: Ensure that the foreign key is selected

When loading associated data using Laravel Eloquent, the associated data may return null if the selected column is specified in the main model query but does not contain a foreign key for establishing the relationship. This article explains the cause of this problem in detail and provides a solution: explicitly include all the foreign keys required for associations in the select statement of the main model query to ensure that Eloquent matches and loads the associated data correctly.

Problem description

In Laravel applications, when trying to preload the associated data (such as Files and User models) through the with() method of Eloquent, and using the select() method in the main model (Course) query, you may encounter the situation where the associated data returns null. Even though the association relationship is correctly defined in the model and foreign keys actually exist in the database, Eloquent cannot establish the correct association map in memory because the main query does not select these foreign key columns.

Consider the following scenario: We want to get the id, courseDisplayName, and aboutCourse columns from the Course table, and preload their associated files (get id, FileName, FilePath) and user (get id, name) information at the same time.

The initial Eloquent query code might look like this:

 $course = Course::with(['files:id,FileName,FilePath','user:id,name'])
    ->select('id','courseDisplayName','aboutCourse')
    ->where('userId',$request->tutorId)
    ->get();

The corresponding Course model defines the belongsTo relationship with Files and User:

 // Course Model
class Course extends Model
{
    public function files()
    {
        return $this->belongsTo(Files::class, 'fileId', 'id');
    }

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

Although the relationship definition is correct, in the output results of the above query, the data associated with files and user are displayed as null:

 [
    {
        "id": 20,
        "courseDisplayName": "asasasb",
        "aboutCourse": null,
        "files": null, // expect data, but null
        "user": null // expect data, but null
    },
    {
        "id": 14,
        "courseDisplayName": "yuu",
        "aboutCourse": "kljkl",
        "files": null,
        "user": null
    }
]

Analysis of the cause of the problem

The root cause of this problem is that when Eloquent preloads the associated data, it needs to rely on foreign key columns in the main model query results to match and link the associated model. When you explicitly use the select() method in the main model query and do not contain foreign key columns for establishing the association (such as fileId and userId), Eloquent cannot obtain these critical connection information.

Specifically, Course::with(['files', 'user']) will first execute the main query to get the Course record. Eloquent then collects the fileId and userId values in these Course records and uses them to perform separate queries (or in some cases JOIN operations) to obtain the corresponding Files and User records. If fileId and userId are not in the result set of the main query, Eloquent cannot build subsequent association queries, nor can the retrieved association data be correctly "injected" into the corresponding Course object.

Solution

The solution to this problem is very straightforward: In the select() statement of the main model, in addition to selecting the desired business data column, all foreign key columns used to establish the association must also be included.

For this example, the Course model associates the Files model through fileId and the User model through userId. Therefore, in the select() method, fileId and userId need to be included as well.

The corrected Eloquent query code is as follows:

 $course = Course::with(['files:id,FileName,FilePath','user:id,name'])
    ->select('id','courseDisplayName','aboutCourse', 'userId', 'fileId') // Add 'userId' and 'fileId'
    ->where('userId',$request->tutorId)
    ->get();

By adding userId and fileId into the select statement, Eloquent can obtain these foreign key values while obtaining the Course record. This way, when Eloquent tries to load files and user associations, it can correctly use these foreign key values to perform subsequent association queries and map the results back to the Course object.

Notes and best practices

  1. Necessity of foreign keys: When you use the select() method for the main model, it is important to remember that any foreign key used to preload associations with() must be included in the column list of select().
  2. Default behavior: If your Eloquent query does not use the select() method, then Eloquent will select all columns in the table by default (SELECT *) and in this case foreign keys will usually be included, so this problem will not occur.
  3. Performance considerations: Although foreign keys are required to solve the association problem, in practical applications, the principle of selecting only necessary columns should still be followed to optimize query performance and reduce memory consumption. Avoid unnecessarily selecting all columns unless you really need them.
  4. Multi-level association: If there is a multi-level association (for example, Course -> Files -> FileOwner), when preloading FileOwner, you also need to ensure that the foreign key used to associate FileOwner in the Files model is selected (if the Files model also uses select()).
  5. Readability and Maintenance: For the readability of the code and future maintenance, it is recommended that all relationship-related foreign key columns be explicitly listed in the select() method, even if they may not be strictly required in some cases (for example, when select() is not used).

Summarize

When using Laravel Eloquent for data queries and association preloading, if the result set of the main model is filtered (that is, the select() method is used), be sure to include all foreign key columns used to establish associations in the list of select(). This is a critical step to ensure that Eloquent correctly recognizes and loads associated data. Following this principle can effectively avoid the problem of empty associated data caused by missing foreign keys, thereby building a robust and efficient Laravel application.

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