Here is a situation: we want to add a mixin with inputs for something like meta-description and meta-keywords to the “site” content and the landing-page content any content that will have a “show” page. But the only way to get the mixin onto the “site” content is to put it in the module.xml file. But then every content type will have it and we don’t want that.
There needs to be a way to add a mixin to a built-in content type.
You could also just call it by name if I recall correctly:
<inline mixin="metadata"/>
The content of your mixin’s <items> node should be this (fieldset added because a content-type mixin won’t add a heading automatically like a module.xml mixin does):
<field-set name="metadata"> IMPORTANT! This name must be unique within the contenttypes using it!
<label>SEO metadata overrides</label>
<items>
<input name="seo-title" type="TextLine">
<label>Override "Title"</label>
<occurrences minimum="0" maximum="1"/>
<help-text>As a default the text in the "Title"-field will be used.</help-text>
</input>
<input name="seo-preface" type="TextArea">
<label>Override "Description"</label>
<occurrences minimum="0" maximum="1"/>
<help-text>As a default the text in the "Preface"-field will be used.</help-text>
</input>
</items>
</field-set>
There are plans to add a filter to the module mixins so they don’t appear on every content on a site. No ETA, so in the meantime they will appear on all.