Is it possible to manipulate Picklist's Field Dependencies with Metadata API? If so, is there any reference or example out there?
You should be able to, but you got to be careful to construct the xml to match what Salesforce expects. The field dependency and values dependency is stored in the object metadata file. For ex., it is stored in opportunity.object
for Opportunity. You can manipulate the data and deploy it back.
Here is an example of configuration in UI:
And how it is represented in Metadata:
Parent Picklist Metadata
<fields>
<fullName>Parent_Picklist__c</fullName>
<externalId>false</externalId>
<label>Parent Picklist</label>
<picklist>
<picklistValues>
<fullName>Foo</fullName>
<default>false</default>
</picklistValues>
<picklistValues>
<fullName>Bar</fullName>
<default>false</default>
</picklistValues>
<picklistValues>
<fullName>Tom</fullName>
<default>false</default>
</picklistValues>
<picklistValues>
<fullName>Jerry</fullName>
<default>false</default>
</picklistValues>
<sorted>false</sorted>
</picklist>
<trackFeedHistory>false</trackFeedHistory>
<trackTrending>false</trackTrending>
<type>Picklist</type>
</fields>
Child Picklist Metadata
<fields>
<fullName>Child_Picklist__c</fullName>
<externalId>false</externalId>
<label>Child Picklist</label>
<picklist>
<controllingField>Parent_Picklist__c</controllingField>
<picklistValues>
<fullName>Bob</fullName>
<controllingFieldValues>Foo</controllingFieldValues>
<controllingFieldValues>Jerry</controllingFieldValues>
<controllingFieldValues>Tom</controllingFieldValues>
<default>false</default>
</picklistValues>
<picklistValues>
<fullName>Alias</fullName>
<controllingFieldValues>Bar</controllingFieldValues>
<controllingFieldValues>Foo</controllingFieldValues>
<controllingFieldValues>Jerry</controllingFieldValues>
<default>false</default>
</picklistValues>
<picklistValues>
<fullName>Fred</fullName>
<controllingFieldValues>Bar</controllingFieldValues>
<controllingFieldValues>Foo</controllingFieldValues>
<controllingFieldValues>Tom</controllingFieldValues>
<default>false</default>
</picklistValues>
<picklistValues>
<fullName>Velma</fullName>
<controllingFieldValues>Bar</controllingFieldValues>
<controllingFieldValues>Jerry</controllingFieldValues>
<controllingFieldValues>Tom</controllingFieldValues>
<default>false</default>
</picklistValues>
<sorted>false</sorted>
</picklist>
<trackFeedHistory>false</trackFeedHistory>
<trackTrending>false</trackTrending>
<type>Picklist</type>
</fields>