[SalesForce] How to get a field’s API name when querying a field of datatype, Metadata Relationship(Field Definition)

Are you updating an existing field? Or creating a new one and adding a value?

The CustomMetadata and the CustomObject are two different Metadata types, but under CustomMetadata you will find the "values of the fields created in the CustomMetadata" and under CustomObject you will find the CustomMetadata object itself.

For instance, a CustomMetadata named "Off Limit Settings" with a number field named "AccountOffLimitMonths" will be shown as a CustomObject like (file: objects/Off_Limit_Settings__mdt.object):

<?xml version="1.0" encoding="UTF-8"?>
<CustomObject xmlns="http://soap.sforce.com/2006/04/metadata">
    <fields>
        <fullName>AccountOffLimitMonths__c</fullName>
        <externalId>false</externalId>
        <fieldManageability>DeveloperControlled</fieldManageability>
        <inlineHelpText>Lorem ipsum dolor</inlineHelpText>
        <label>Account Off Limit Months</label>
        <precision>18</precision>
        <required>false</required>
        <scale>0</scale>
        <type>Number</type>
        <unique>false</unique>
    </fields>
    <label>Off Limit Settings</label>
    <pluralLabel>Off Limit Settings</pluralLabel>
    <visibility>Public</visibility>
</CustomObject>

And the field will be shown under CustomMetadata as (file: customMetadata/Off_Limit_Settings.Off_Limit.md):

<?xml version="1.0" encoding="UTF-8"?>
<CustomMetadata xmlns="http://soap.sforce.com/2006/04/metadata" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
    <label>Off Limit</label>
    <protected>false</protected>
    <values>
        <field>AccountOffLimitMonths__c</field>
        <value xsi:type="xsd:double">12.0</value>
    </values>
</CustomMetadata>

If you want to add a new field, modify the first file and deploy it in a package. If you want to modify a field, use the second file.

You still need to use JSON construction if you want to set parent attributes without a true query for the records.

Non Functional Code

EntityDefinition sourceObject = [SELECT QualifiedApiName FROM EntityDefinition LIMIT 1];
MyMapping__mdt mapping = new MyMapping__mdt(
    Source_Object__r = sourceObject
);

Error

Field is not writeable: MyMapping__mdt.Source_Object__r

Functional Code

Map<String, Object> mockMapping = new Map<String, Object>
{
    'attributes' => new Map<String, Object>
    {
        'type' => 'MyMapping__mdt '
    },
    'Source_Object__r' => new Map<String, Object>
    {
        'attributes' => new Map<String, Object>
        {
            'type' => 'EntityDefinition'
        },
        'QualifiedApiName' => 'Account'
    }
};
MyMapping__mdt mapping = (MyMapping__mdt)JSON.deserialize(
    JSON.serialize(mockMapping), MyMapping__mdt.class
);
system.debug(mapping.Source_Object__r.QualifiedApiName);

Log

Account