How do I add a value to a tag field type using the entries service saveEntry method?

Question

I am updating an entry using the entries service, I want to add a tag value into a tag field type (classifiedTags).

$entry = craft()->entries->getEntryById(120);
$entry->getContent()->classifiedTags = 'tagtest';
$entry->title = 'Updating title to this';
$entry->getContent()->description = 'this is the new description';
craft()->entries->saveEntry($entry);

It runs and updates everything except the tag field (classifiedTags).

If I change it from a string to array like below:

$entry->getContent()->classifiedTags = array('tagtest');

It gives me this error:

CDbCommand failed to execute the SQL statement: SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (ggmg_craft.craft_relations, CONSTRAINT craft_relations_targetId_fk FOREIGN KEY (targetId) REFERENCES craft_elements (id) ON DELETE CASCADE). The SQL statement executed was: INSERT INTO craft_relations (fieldId, sourceId, sourceLocale, targetId, sortOrder, dateCreated, dateUpdated, uid) VALUES (:row0_col0, :row0_col1, NULL, :row0_col3, :row0_col4, :row0_col5, :row0_col6, :row0_col7). Bound with :row0_col0='43', :row0_col1='31823', :row0_col3='tagtest', :row0_col4=1, :row0_col5='2016-07-14 21:16:04', :row0_col6='2016-07-14 21:16:04', :row0_col7='d2228ed9-cc60-478d-929c-98efd776b63c'

Which leads me to believe I am getting warmer.

Craft CMS v 2.6.2796 PHP v 5.6.13

Answer 1

The Tags field classifiedTags needs an array of tag IDs, not slugs/titles. Also, if this is an existing entry and you want to retain tags already added to the field, you'll need to pull the existing tag IDs using $entry->classifiedTags->ids() and merge with those.

$tag = craft()->elements->getCriteria(ElementType::Tag, [
    'groupId' => 1,
    'slug' => 'tagtest',
])->first();
if ($tag) {
  $existingTags = $entry->classifiedTags->ids();
  $entry->getContent()->setAttribute('classifiedTags', array_merge($existingTags, [$tag->id]));
}

Note that if the tag doesn't already exist, you'll need to create it using the TagsService:

$tag->groupId = 1;
$tag->getContent()->title = 'tagtest';
$success = craft()->tags->saveTag($tag);