Scriptrunner script to query field with username and set that to assignee

unknown

Fix denoted below. 

PD Sheehan

Will your custom field with user name be a User Picker or a Free Text field?

Either way, you should be able to just read that field's value with something like :

import com.atlassian.jira.component.ComponentAccessor
def cf = ComponentAccessor.customFieldManager.getCustomFieldObjectByName('Some Field with a user')
def user 
if(cf.cusomtFieldType == 'com.atlassian.jira.plugin.system.customfieldtypes:userpicker'){
  user = issue.getCustomFieldValue(cf)
} else {
  def userName = issue.getCustomFieldValue(cf)
  user = ComponentAccessor.userManager.getUserByName(userName)
}
issue.setAssignee(user)

I wouldn't actually write it this way -- with the "if".  You should already know which type your customer field is. But this should show you both ways to get a user object that can then be applied to the issue assignee field

And if you are doing this in a workflow post function, make sure that the script is before the "Update change history for an issue and store the issue in the database" step.