How to make a cascading field second level required only if second level there.

Teja, for your case where items might be childless... When the HashMap only has only selected item, you can pass its ID to OptionsManager.findByParentId(Long).  If that's not empty, it means the item has children that the user should have selected so you can make your validator fail.

How to make a cascading field second level required only if second level there.

A

A1, A2

B

B1, B2

C

No second level

I tried to but is not working somehow.
no error and the transition pass.
I tried to have a simple script-runner validator and it works.
The code is : cfValues[}}{{"The name of your customfield"}}{{]?.keySet()?.size() == 2
https://scriptrunner.adaptavist.com/4.3.4/jira/recipes/workflow/validators/simple-scripted-validators.html#_validating_cascading_selects 

Try using this slight variation:

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.MutableIssue

def issue = issue as MutableIssue
def cascadingSelect = ComponentAccessor.customFieldManager.getCustomFieldObjects(issue)?.findByName("Cascading Field Name")
def values = issue.getCustomFieldValue(cascadingSelect) as HashMap

// Will return true if both the parent and child options in the Cascading Select have a value
def keySetSize = values?.keySet()?.size()
keySetSize == 2

I have added the last link in the workflow in the creation field and somehow the code is not working for me.
The script is not showing any error in the logs but the transition pass

I used the code posted by @David Lawless, and it worked fine. No errors. Just input your custom field name here: findByName("CascadingSelect")

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.MutableIssue

def issue = issue as MutableIssue
def cascadingSelect = ComponentAccessor.customFieldManager.getCustomFieldObjects(issue)?.findByName("CascadingSelect")
def values = issue.getCustomFieldValue(cascadingSelect) as HashMap

// Will return true if both the parent and child options in the Cascading Select have a value
values?.keySet()?.size() == 2

Using Jira 7.10 I have been trying to get the ScriprtRunner workaround referenced above, namely adding

"cfValues.get['your_ cascading_field_name']?.key.set()?.size() == 2" as a simple scripted validator.

https://scriptrunner.adaptavist.com/4.3.4/jira/recipes/workflow/validators/simple-scripted-validators.html#_validating_cascading_selects

However I found this returned the following error - 

[Static Type Checking] - Cannot find matching java.lang.Object#keyset(). Please check if the declared type is correct and if the method exists. Possible solutions: getAt(java.lang.String) @ line 1, column 1.

I raised a ticket with Adaptavist and was told "Static type checking errors can sometimes be ignored, and this is one of those cases. The code should still work fine, even though an error is shown. That being said, you can try using this code instead and seeing if that gets rid of the error:

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.MutableIssue

def issue = issue as MutableIssue
def cascadingSelect = ComponentAccessor.customFieldManager.getCustomFieldObjects(issue)?.findByName("CascadingSelect")
def values = issue.getCustomFieldValue(cascadingSelect) as HashMap

// Will return true if both the parent and child options in the Cascading Select have a value
values?.keySet()?.size() == 2

I haven't tried ignoring the static type error, but have tried the code provided and it worked a treat.  Hopefully may help someone

spent unnecessary cycles on this issues.  Atlassian is not providing enough details as to why child filed is not becoming mandatory.   Their answer is to purchase ScriptRunner is not acceptable.   Why are they even allowing Cascading select fields if they have this many issues.

Does the description to this ticket mean that this defect will never be in scope nor actually resolved?