How to set assignee from user type custom field?

[Jk 140401]

Hi Team,

i am trying to set Assignee value from custom type user field , but i am getting value in XXXXX(XXXXX) format.

When i am trying to set the value using issue.setAssignee() i am getting null value.

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.Issue
import com.atlassian.jira.issue.MutableIssue
import com.atlassian.jira.issue.customfields.option.LazyLoadedOption
import com.atlassian.jira.issue.fields.CustomField
import com.atlassian.jira.user.ApplicationUser;
import com.atlassian.jira.user.util.UserManager
import com.atlassian.crowd.embedded.api.User
import org.apache.log4j.Logger
import org.apache.log4j.Level

def log = Logger.getLogger(" ")
log.setLevel(Level.DEBUG)

def customFieldManager = ComponentAccessor.getCustomFieldManager()
def user = ComponentAccessor.getJiraAuthenticationContext().getLoggedInUser()
UserManager userManager = ComponentAccessor.getUserManager();

def managerId = customFieldManager.getCustomFieldObjectByName("Dept Approver's(Manager) Id")
String  managerIdvalue =issue.getCustomFieldValue(managerId)

ApplicationUser Mgruser = userManager.getUserByName(managerIdvalue)

log.info "managerId: "+managerId
log.info "managerIdvalue: "+managerIdvalue
log.info "Mgruser : "+Mgruser

issue.setAssignee(Mgruser )
//issue.setAssignee(Mgruser )

==========OUT PUT============

2017-08-09 10:50:20,455 INFO [ ]: managerId: Dept Approver's(Manager) Id
2017-08-09 10:50:20,456 INFO [ ]: managerIdvalue: 44018514(44018514)
2017-08-09 10:50:20,456 INFO [ ]: Mgruser : null

issue.setAssignee(Mgruser) --> should assign the value of "Mgruser " into Assignee.

but "Mgruser" is getting null value.

please advise how to get username from coustomfield and  set it to Assignee default field.

[Nic Brough _Adaptavist_]

getUserByName expects to be passed a user name, which in JIRA-speak means the login id.  Do you really have a user who logs in with that number as their login?

[Jk 140401]

yes, we have users with username as numerical ids and full name as usual. i need to assign user with this numerical id.