Codeigniter join 2 tables data with new column for id value - php

Question

I'm new to Codeigniter and PHP, just learning. I've searched for this but couldn't find the right answer. I have a table that contain some data, lets call it posts and in this table, each post have a category_id. I have another table called categories and in this table I have 2 columns: id and name.
I want to join the two tables that the result I'll get is the same as posts with another column called category_name that will be taken from categories.name. Just can't figure out how to do so.
This is what I was up to so far:
function getPostsWithByCategoryID($numberOfRows, $start, $categoryId)
{
$this->db->select('*');
$this->db->from('posts')->order_by('id','desc')->limit($numberOfRows, $start);
$this->db->where('category_id', $categoryID);
$this->db->join('categories', 'posts.category_id == categories.id');
$query = $this->db->get();
return $query->result_array();
}
Thanks in advance.
========================================================================
EDIT:
After trying Adrian Forsius answer:
function getPostsWithByCategoryID($numberOfRows, $start, $categoryId)
{
$this->db->select('*, category.name AS category_name');
$this->db->from('posts')->order_by('id','desc')->limit($numberOfRows, $start);
$this->db->where('category_id', $categoryID);
$this->db->join('categories', 'posts.category_id = categories.id');
$query = $this->db->get();
return $query->result_array();
}
I'm getting this error:
Error Number: 1054: Unknown column 'category.name' in 'field list'
SELECT *, `category`.`name` AS category_name
FROM (`posts`)
JOIN `categories` ON `posts`.`category_id` = `categories`.`id`
WHERE `category_id` IS NULL
ORDER BY `id` desc
LIMIT 20
====
EDIT 2:
So I found some mistakes in the code and fixed them, tried again, but still I get an error: (The mistakes were using $categoryID instead of $categoryId and select category.name instead of the true name of the table categories which is categories.name
Error Number: 1052: Column 'id' in order clause is ambiguous
SELECT *, `categories`.`name` AS category_name
FROM (`posts`)
JOIN `categories` ON `posts`.`category_id` = `categories`.`id`
WHERE `category_id` = '3'
ORDER BY `id` desc
LIMIT 20

Answer 1

This should do what you are looking for:
function getPostsWithByCategoryID($numberOfRows, $start, $categoryId)
{
$this->db->select('*, categories.name AS category_name');
$this->db->from('posts')->order_by('posts.id','desc')->limit($numberOfRows, $start);
$this->db->where('category_id', $categoryId);
$this->db->join('categories', 'posts.category_id = categories.id');
$query = $this->db->get();
return $query->result_array();
}

Answer 2

replace
$this->db->select('*');
by
$this->db->select('posts.*');
$this->db->select('categories.name','category_name');
CI's documentation is pretty good.

Answer 3

Try this I think you forget to place = in place of ==
function getPostsWithByCategoryID($numberOfRows, $start, $categoryId)
{
$this->db->select('*');
$this->db->from('posts')->order_by('id','desc')->limit($numberOfRows, $start);
$this->db->where('category_id', $categoryID);
$this->db->join('categories', 'posts.category_id = categories.id');
$query = $this->db->get();
return $query->result_array();
}