How to save a new address attribute in checkout - magento2

Question

I am trying to save geolocation along with the customer address.
I have added Let & Lng using the install script
$customerSetup->addAttribute('customer_address', 'latitude', [
'type' => 'varchar',
'label' => 'Latitude',
'input' => 'text',
'required' => false,
'visible' => true,
'visible_on_front' => true,
'user_defined' => false,
'sort_order' => 43,
'position' => 43,
'system' => 0,
]);
$attributeLat = $customerSetup->getEavConfig()->getAttribute('customer_address', 'latitude')
->addData([
'attribute_set_id' => $attributeSetId,
'attribute_group_id' => $attributeGroupId,
'used_in_forms' => ['adminhtml_customer_address', 'customer_address_edit', 'customer_register_address','customer_address'],
]);
$attributeLat->save();
//latitude - End
$customerSetup->addAttribute('customer_address', 'longitude', [
'type' => 'varchar',
'label' => 'Longitude',
'input' => 'text',
'required' => false,
'visible' => true,
'visible_on_front' => true,
'user_defined' => false,
'sort_order' => 43,
'position' => 43,
'system' => 0,
]);
$attributeLng = $customerSetup->getEavConfig()->getAttribute('customer_address', 'longitude')
->addData([
'attribute_set_id' => $attributeSetId,
'attribute_group_id' => $attributeGroupId,
'used_in_forms' => ['adminhtml_customer_address', 'customer_address_edit', 'customer_register_address','customer_address'],
]);
$attributeLng->save();
//longitude - End}
I can update values in the admin backend without an any issue.
In the checkout page form fields for "latitude" & "longitude" are appearing. But values don't save along with the customer address.
I am using Magento CE 2.2.3

Answer 1

You need to add etc/extension_attributes.xml for Magento\Customer\Api\Data\AddressInterface:
<extension_attributes for="Magento\Customer\Api\Data\AddressInterface">
<attribute code="longitude" type="string" />
</extension_attributes>
Add etc/fieldset.xml:
<fieldset id="sales_convert_quote_address">
<field name="longitude">
<aspect name="to_customer_address" />
<aspect name="to_order_address" />
</field>
</fieldset>
Add plugin in di.xml for Magento\Customer\Model\Address where you need to save your custom attribute in the beforeUpdateData function
<type name="Magento\Customer\Model\Address">
<plugin disabled="false" name="vendor_plugin_quote_model_address" sortOrder="10"
type="Vendor\Module\Plugin\Customer\Model\Address"/>
</type>
public function beforeUpdateData(
\Magento\Customer\Model\Address $subject,
\Magento\Customer\Api\Data\AddressInterface $address
)
And then you should be able to see your attribute saved on the Customer Address.