Using Services to get all the Categories related to an Entry?

Question

In a plugin, I've been advised to try to use craft built-in services instead of direct db queries. So can anyone help with this: For an entry type that includes a categories field, how to get all the categories related to an entry?

I found this:

$entry = craft()->entries->getEntryById($entryId);

So I suppose this might work as well:

$category = craft()->categories->getCategoryById($categoryId);

But I can't find anything intelligible to me under the relations service.

Thanks for any help.

EDIT: On a related note, I see here that in twig you can do this:

{% for entry in craft.entries.section('news').limit(10) %}
<article>
    <h1><a href="{{ entry.url }}">{{ entry.title }}</a></h1>
    {{ entry.summary }}
    <a href="{{ entry.url }}">Continue reading</a>
</article>
{% endfor %}

Whereby Craft/Twig does all the work to find and return the 'summary' field related to each entry. Is there any php statement like this next confused one, following which I could get the categories related to each event?

$events = craft()->entries.section('events').start('<=$toDateYmd').end('OR', '>=$fromDateYmd', '').status('publish');
foreach ($events as $event)
   $categories = $event.category;

Thank you!

Answer 1

If you have an EntryModel and know the category field's handle, you don't really need a service:

$categories = $entry->categoryFieldHandle;

In other cases, you can construct an ElementCriteriaModel to search for related elements (categories, entries, users or assets), using a number of different variables. Easy example to get related categories from an entry ID:

$criteria = craft()->elements->getCriteria(ElementType::Category);
$criteria->relatedTo = array($entryId);
$categories = craft()->elements->findElements($criteria);

Answer 2

OK, here is what I worked out, through hours of trial and error. A hint came from the docs:

This should be the starting point any time you want to fetch elements in Craft.

$criteria = craft()->elements->getCriteria(ElementType::Entry);

$criteria->section = 'news';

$entries = $criteria->find();

In my plugin's services file this produces the desired results:

// Set up the criteria
$criteria = craft()->elements->getCriteria(ElementType::Entry);
$criteria->section = 'events';
$criteria->startDate = array("<=$toDateYmd", NULL);
$criteria->endDate = array(">=$fromDateYmd", NULL);

$entries = $criteria->find();  //Do the query

foreach ($entries as $entry) {
    $categories = $entry->categories;
    echo "<br>title: $entry->title Start: $entry->startDate<br><pre>";
    echo "Categories:</br>";
    foreach ($categories as $k => $v) echo " [$k] => $v<br>";
}

When adding the criterias, each array is basically doing an 'OR'. So it's if startDate <= $toDateYmd OR startdate = NULL. I just happen to need to accept NULL as a value here.

If anyone has a better way of doing it, please let me know, but code-wise this seems quite concise.