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ScriptRunner allows you to script specific ScriptRunner fields. As an example, if you move an issue to a different project, a new key is assigned. Jira is smart enough that it keeps the old key as well, so if you search for the old issue key, you will be redirected to the new issue key. If for some reason you'd like to find out what the previous key is, you can script this with a ScriptRunner field. Following code can be used in for the Script Field:

import com.atlassian.jira.issue.IssueManager;
import com.atlassian.jira.component.ComponentAccessor;
 
IssueManager issueManager = ComponentAccessor.getIssueManager();
Set<String> tickets = issueManager.getAllIssueKeys(issue.id)
tickets = tickets.minus(issue.key)
if (tickets.isEmpty()) {
   return "N/A"
}
return tickets.join(', ')

import com.atlassian.jira.issue.Issue;
import com.atlassian.jira.issue.IssueManager;
import com.atlassian.jira.issue.MutableIssue;
import com.atlassian.jira.component.ComponentAccessor;
 
void setSummary(String key,String newSummary) {
IssueManager im = ComponentAccessor.getIssueManager();
        MutableIssue issue = im.getIssueObject(key);
        issue.setSummary(newSummary);
        issue.store();
}
 
setSummary("CD-4","Insert your Summary");

import com.atlassian.jira.component.ComponentAccessor
import static com.atlassian.jira.issue.IssueFieldConstants.*
 
def changedItem = ComponentAccessor.getChangeHistoryManager().getAllChangeItems(issue)?.findAll {it.field == "testRadio"}?.last()
 
return "${changedItem?.getUserKey()}"

Reference: Pull User Details for field data change