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Employee not found with id null grails
class Employee {

    Integer empId
    String firstName
    String lastName

    static constraints = {
        empId()
        firstName()
        lastName()
    }

    static mapping = {
        id generator:'assigned', name:'empId'
        version false
    }
}

static constraints = {
    empId()
    firstName()
    lastName()
}

static mapping = {
    id generator:'assigned', name:'empId', column: 'emp_id'
    version false
}

public void setEmpId(Long empId){
    this.empId = empId
    this.id = empId
}

id name: 'customId', generator: 'sequence', params: [sequence:'some_sequence']