ValueError: Expected singleton: daily.attendance.line(123,124,125

@api.model
def create(self, vals):
    curr = datetime.now()
    new_date = datetime.strftime(curr, '%Y-%m-%d')
    cal_obj = self.env['daily.attendance'].search([])

@api.constrains('date')
def _date_test_unique(self):
     for rec in self:
        if self.search_count([('date', '=', rec.date)]) > 1:
            raise ValidationError(_('Current Date Attendance Already Existed!'))
          
@api.onchange('user_id')
def onchange_department(self):
    if self.user_id == True:
        emps = self.env['hr.employee'].search([])
        emp_attd = []
        from datetime import datetime

        now = datetime.now() # current date and time
        check_in = now.strftime('%Y-%m-%d %H:%M:%S')
        check_in_from = now.strftime('%Y-%m-%d 05:30')
        check_out = now.strftime('%Y-%m-%d %H:%M:%S')
        check_out_from = now.strftime('%Y-%m-%d 14:30')
        for emp in emps:
            vals = {
                'employe_id':emp.id,
                'check_in': check_in_from,
                'check_out': check_out_from,
                'is_present': True
            }
            emp_attd.append([0, 0, vals])
        self.update({
            'employee_ids': emp_attd,
        })
    else:
        self.employee_ids = False
    return {
        'type': 'ir.actions.client',
        'tag': 'reload',
    }

Solution

The error happens when Odoo tries to get employee_ids value from a record set but it expects a record.

for emp in self.employee_ids:

You need to loop over self then access employee_ids field value for each record:

Example:

def attendance_validate(self):
    for rec in self:
        for emp in rec.employee_ids:

You should move the following code outside the for loop

self.write({'state': 'validate', })

Example:

hr_attendance = self.env['hr.attendance']
for rec in self:
    for emp in rec.employee_ids:
        if emp.is_present == True:
            attd_crete_id = hr_attendance .create({'employee_id': emp.employe_id.id,
                                         'check_in': emp.check_in,
                                         'check_out': emp.check_out,
                                         })
    rec.write({
        'state': 'validate',
    })
    ...

Probably you need to call write state to validate when the attendance_validate method succeed (at the end)

improvement:

The following expression

  if emp.is_present == True:

can be simplified to:

  if emp.is_present:

You are using two fields is_present and is_absent, you can simply use is_present and when its value is False (not set) the employee is absent.
You need to remove the second if statement, which is useless
```
elif emp.is_absent == True:
    if emp.is_absent == True:
```

Avoid raising a validation error in the create method because it will break the workflow, instead you can define a constraint on date field:

  @api.constrains('date')
  def _date_test_unique(self):
      for rec in self:
          if self.search_count([('date', '=', rec.date)]) > 1:
              raise ValidationError(_('Current Date Attendance Already Existed!'))

Update:

The create method should return the newly created record:

@api.model
def create(self, vals):
    res = super(CLASS_NAME, self).create(vals)
    # Your code
    return res