Adding the different coding steps

step4/LinkPeer.php

@@ -0,0 +1,22 @@
+<?php
+
+...
+    private function getCollection()
+    {
+        // if the collection is not already cached
+        if (empty($this->collection)) {
+
+            // selects the collection if it exists
+            $this->collection = $this->db->selectCollection(self::COLLECTION);
+
+            // or creates a new one if it doesn't
+            if (empty($this->collection)) {
+                $this->collection = $this->db->createCollection(
+                    self::COLLECTION
+                );
+            }
+        }
+
+        return $this->collection;
+    }
+...

step5/LinkPeer.php

@@ -0,0 +1,31 @@
+<?php
+
+    ...
+
+    public function getIdFromDataset(array $data)
+    {
+        if (!isset($data[self::DB_ID_FIELD])) {
+            return false;
+        }
+        return $data[self::DB_ID_FIELD];
+    }
+
+    public function translateDataset(array $data, $toDb = true)
+    {
+        $keyFrom = self::ID_FIELD;
+        $keyTo = self::DB_ID_FIELD;
+        if (!$toDb)  {
+            $keyFrom = self::DB_ID_FIELD;
+            $keyTo = self::ID_FIELD;
+        }
+
+        if (!isset($data[$keyFrom])) {
+            return false;
+        }
+
+        $data[$keyTo] = $data[$keyFrom];
+        unset($data[$keyFrom]);
+        return $data;
+    }
+
+    ...

step6/LinkPeer.php

@@ -0,0 +1,21 @@
+<?php
+
+    ...
+
+    public function insert(Link $link)
+    {
+        return $this->update($link);
+    }
+
+    public function update(Link $link)
+    {
+        $data = $this->translateDataset($link->toArray(), true);
+
+        if ($data && $this->getCollection()->save($data)) {
+            return true;
+        }
+
+        return false;
+    }
+
+    ...

step7/LinkPeer.php

@@ -0,0 +1,47 @@
+<?php
+
+    ...
+
+    public function fetchAll()
+    {
+        $results = array();
+
+        // perform a find with a blank query
+        foreach ($this->getCollection()->find() as $result) {
+            // Create a Link out of each of the results
+            $results[] = $this->factory($result);
+        }
+
+        return $results;
+    }
+
+    public function fetchByUrl($url)
+    {
+        $query = array('url' => $url);
+        $mongoQuery = $this->translateDataset($query);
+        $response = $this->getCollection()->findOne(
+            $mongoQuery
+        );
+
+        // if the database returns a result return the link
+        if ($response) {
+            return $this->factory($response);
+        }
+
+        // Otherwise return null
+        return null;
+    }
+
+    private function factory(array $linkArray)
+    {
+        if (!isset($linkArray['_id'])) {
+            throw new Exception('Missing data');
+        }
+
+        $linkArray = $this->translateDataset($linkArray, false);
+
+        $tmp = new Link();
+        return $tmp->fromArray($linkArray);
+    }
+
+    ...

step8/Link.php

@@ -0,0 +1,85 @@
+<?php
+require_once(dirname(__FILE__) . '/LinkPeer.php');
+
+class Link {
+    private $url;
+    private $tags = array();
+
+    public function __construct()
+    {
+        // Not required to do anything at this time.
+    }
+
+    public function getUrl()
+    {
+        return $this->url;
+    }
+
+    public function getTags()
+    {
+        return $this->tags;
+    }
+
+    public function setUrl($url)
+    {
+        $this->url = (string) $url;
+    }
+
+    public function addTag($tag)
+    {
+        $this->tags[] = (string) $tag;
+    }
+
+    public function removeTag($tag)
+    {
+        $success = false;
+        // look through the current tags for the specified tag
+        foreach ($this->tags as $key => $tagName) {
+            if ($tagName == $tag) {
+                // remove the tag from the list
+                unset($this->tags[$key]);
+                $success = true;
+            }
+        }
+        return $success;
+    }
+
+    public function __toString()
+    {
+        // example: http://www.google.com/ [usa, search]
+        return sprintf('%s [%s]', $this->url, implode(', ', $this->tags));
+    }
+
+    public function toArray()
+    {
+        return array(
+            // _id is the name of the url in this case as it makes it easier
+              // for interoperability with the Database
+            'url' => $this->url,
+            'tags' => $this->tags,
+        );
+    }
+
+    public function fromArray($link)
+    {
+        // Ensure the url is set
+        if (empty($link['url']) || !is_string($link['url'])) {
+            throw new Exception('Incorrect data supplied');
+        }
+
+        $this->url = $link['url'];
+
+        // Only set tags if they conform to our structure
+        $tags = array();
+        if (!empty($link['tags']) && is_array($link['tags'])) {
+            foreach ($link['tags'] as $tag) {
+                if (is_string($tag)) {
+                    $tags[] = $tag;
+                }
+            }
+        }
+        $this->tags = $tags;
+
+        return $this;
+    }
+}

step8/LinkPeer.php

@@ -0,0 +1,30 @@
+<?php
+
+    ...
+
+    public function delete(Link $link)
+    {
+        $link = $this->translateDataset($link->toArray(), true);
+
+        $linkKey = $this->getIdFromDataset($link);
+        $key = array(
+            '_id' => $linkKey,
+        );
+
+        if ($linkKey) {
+            $options = array(
+                'fsync' => true, // Forces the update to be synced to disk
+            );
+            try {
+                if ($this->getCollection()->remove($key, $options)) {
+
+                    return true;
+                }
+            } catch (MongoException $e) {
+                // @TODO log the error
+            }
+        }
+        return false;
+    }
+
+    ...