Use system value is not showing up

Question

Hi I am new to magento2 and learning currently the basics.

I have an module and set up some system configuration for it but the checkbox Use system value is not showing up next to the field. I have setup a default value for it in config.xml. I am using magento-ce 2.2.5

Expected:

Actual:

Here is my system.xml:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
    <system>
   <tab id="mastering" translate="label" sortOrder="10000">
        <label>Mastering</label>
    </tab>
    <section id="mastering" translate="label" sortOrder="10" showInDefault="1" showInWebsite="0" showInStore="0">
        <label>Mastering</label>
        <tab>mastering</tab>
        <resource>Mastering_SampleModule::mastering</resource>
        <group id="general" translate="label" sortOrder="10" showInDefault="1" showInWebsite="0" showInStore="0">
            <label>Item creation by schedule</label>
            <field id="enabled" translate="label" type="select" sortOrder="10" showInDefault="1" showInWebsite="0" showInStore="0">
                <label>Enabled</label>
                <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
            </field>
            <field id="cron_expression" translate="label" type="text" sortOrder="20" showInDefault="1" showInWebsite="0" showInStore="0">
                <label>Add Item Cron Expression</label>
            </field>
        </group>
    </section>
</system>

My config.xml:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Store:etc/config.xsd">
<default>
    <mastering>
        <general>
            <enabled>0</enabled>
            <cron_expression>* * * * *</cron_expression>
        </general>
    </mastering>
</default>

Answer 1

I believe the trick to having that checkbox is to add the canRestore="1" attribute to your config option. i.e.

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
    <system>
   <tab id="mastering" translate="label" sortOrder="10000">
        <label>Mastering</label>
    </tab>
    <section id="mastering" translate="label" sortOrder="10" showInDefault="1" showInWebsite="0" showInStore="0">
        <label>Mastering</label>
        <tab>mastering</tab>
        <resource>Mastering_SampleModule::mastering</resource>
        <group id="general" translate="label" sortOrder="10" showInDefault="1" showInWebsite="0" showInStore="0">
            <label>Item creation by schedule</label>
            <field id="enabled" translate="label" type="select" sortOrder="10" showInDefault="1" showInWebsite="0" showInStore="0">
                <label>Enabled</label>
                <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
            </field>
            <field id="cron_expression" translate="label" type="text" sortOrder="20" showInDefault="1" showInWebsite="0" showInStore="0" canRestore="1">
                <label>Add Item Cron Expression</label>
            </field>
        </group>
    </section>
</system>