/**
* Adds an attribute field.
*
* @param AttributeMetadata $attribute
* @return self
* @throws MetadataException If the attribute key already exists as a relationship.
*/
public function addAttribute(AttributeMetadata $attribute)
{
if (isset($this->relationships[$attribute->getKey()])) {
throw MetadataException::fieldKeyInUse('attribute', 'relationship', $attribute->getKey(), $this->type);
}
$this->attributes[$attribute->getKey()] = $attribute;
ksort($this->attributes);
return $this;
}
