Primary category

Saved in post meta data as: _yoast_wpseo_primary_{taxonomy-name}

• Register post meta for all taxonomies (class-wpseo-meta.php)
• Add primary_{taxonomy_name} values to ScriptData under metaData property (class-post-metabox-formator.php)
• Use MetaboxFieldSync onChange in packages/js/src/components/PrimaryTaxonomyPicker.js
• Fix the way we get primary category in packages/js/src/initializers/primary-category.js

• primary_{taxonomy}

Reducer

• packages/js/src/redux/reducers/primaryTaxonomies.js

Initializer

• packages/js/src/initializers/primary-category.js

Update event listener

• packages/js/src/analysis/PostDataCollector.js

Components

• packages/js/src/components/PrimaryTaxonomyPicker.js

Initial state

• packages/js/src/redux/initial-state/primaryTaxonomies.js

Register metadata

• inc/class-wpseo-meta.php

Tests

• packages/js/tests/helpers/fields/getPrimaryTerms.test.js

Test instuctions

• Make sure you have 2-3 categories for posts
• Edit a post and select the primary category and save
• Refresh and check the primary category was saved.
• Repeat the test with classic editor.

Inspect the the page and look for an input with the id yoast_wpseo_primary_category. Manually change it's value to text, zero, or a negative number and save.
Refresh the page and inspect the page.

• Check the text value is not there.

Without the metabox

• Comment out line 81 in class-metabox.php:

// add_action( 'add_meta_boxes', [ $this, 'add_meta_box' ] );

• Make sure you have 2-3 categories for posts
• Edit a post and select the primary category and save
• Refresh and check the primary category was saved.