CodeIgniter报错：在布尔值上调用num_rows()方法求助

Let's break down why you're hitting this error and how to fix it step by step.

What's Causing the Error? #

The Call to a member function num_rows() on boolean error happens because your database query is failing. When CodeIgniter's $this->db->get() encounters a broken SQL query, it returns false (a boolean) instead of a valid result set object. Trying to call num_rows() on that boolean value throws the error you see.

Step 1: Debug the Generated SQL #

First, let's see exactly what SQL CodeIgniter is trying to run. Add these lines right before $this->db->get() in your method:

echo $this->db->last_query();
die();

Copy the output and run it directly in your database client (like phpMyAdmin or MySQL Workbench). This will show you the exact syntax error or missing table/field causing the query to fail.

Step 2: Fix the Query Issues #

Looking at your code, there are two key adjustments needed:

1. Disable Automatic Escaping for Complex Queries: CodeIgniter's query builder automatically escapes identifiers, which can break subqueries like yours. Add false as the second parameter to $this->db->select() to prevent this.
2. Add Error Handling: Check if the query succeeded before calling num_rows().

Here's the revised version of your method:

public function out_of_stock_count(){
    // Disable auto-escaping to preserve subquery syntax
    $this->db->select("
        b.manufacturer_name,
        a.product_name,
        a.generic_name,
        a.strength,
        (
            (SELECT IFNULL(SUM(quantity), 0) FROM product_purchase_details WHERE product_id = a.product_id)
            -
            (SELECT IFNULL(SUM(quantity), 0) FROM invoice_details WHERE product_id = a.product_id)
        ) AS stock
    ", false);
    
    $this->db->from('product_information a');
    $this->db->join('manufacturer_information b', 'b.manufacturer_id = a.manufacturer_id', 'left');
    $this->db->group_by('a.product_id');
    $this->db->having('stock < 10');
    
    $query = $this->db->get();
    
    // Handle query failure gracefully
    if (!$query) {
        // Print database error for debugging (remove in production)
        echo $this->db->error()['message'];
        return 0; // Return a safe default or throw an exception
    }
    
    return $query->num_rows();
}

Additional Checks #

• Verify that all tables referenced (product_information, manufacturer_information, product_purchase_details, invoice_details) exist in your database.
• Double-check that all field names (like product_id, quantity, manufacturer_id) are spelled correctly across all tables.
• If you're using a case-sensitive database (like PostgreSQL on Linux), ensure table/field names match the exact casing in your database.

内容的提问来源于stack exchange，提问作者Ehsan