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Use case
Set the assignee based on a User Picker field. If this field is empty, set the reporter as assignee.
Configuration
Add the Update field based on rules to the desired transition or create a new transition.
You can add the post function to the Create transition of the workflow so that the assignee is automatically set every time that an issue is created.
Source field
Choose Summary
Although this field is not relevant in this use case, it is required to choose one. We will set it to Summary.
Target field
Select Assignee
Rules
First rule
Select Logical expression
Expression
Use the following expression after replacing the field code %{issue.cfaaaaa} with the one for the User Picker field.
%{issue.cfaaaaa} = null
Target field value
Enter the following field code.
%{issue.reporter}
Second rule
Select Logical expression
Expression
Use the following expression after replacing the field code %{issue.cfaaaaa} with the one for the User Picker field.
%{issue.cfaaaaa} != null
Enter the field code for the User Picker field.
%{issue.cfaaaaa}
Related use cases
STAFF PICKUse case JWT feature Workflow function Parser functions Label Add user to field depending on selected options 
Set assignee based on priority 
priority() Set assignee depending on issue type issueType() Set issue security level depending on reporter issueSecurityLevel() Set reporter as assignee if a User Picker field is empty 
If you still have questions, feel free to refer to our support team.
