No, you cannot have the ID of the record if it is not created yet.

I don't know why you need the ID but if you want to access the value of the other fields, you can use onchange method to get the value of the fields and do some operations.

@api.onchange('partner_id')
def onchange_partner(self):
    self.partner_id
    self.my_field1
    self.my_field2
    # You can access the value of the fields in the onchange like this

self.id is returning false because it is defined only after creation. An alternative idea to get the id before saving is find the last id of the models and calculate the next one.
For ex:
search_ids = self.pool.get('your.model').search(cr, uid, [])
last_id = search_ids and max(search_ids)

There are two additional ways I can think of:

- use models.NewId: This doesn't give you an actual ID (integer) but depending on your code you can use it as a placeholder.

Usage:

if isinstance(self.id, models.NewId):
    # your code

(note that in this case self.id does not exist in the db yet, but you can imagine it is already "pre-ordered" in the cache)

- It helps to know that Odoo assigns IDs in a strictly linear way: The next record will always have the ID of the previously created record plus one. I guess it is a bit of a cheaty way to work with this fact & it is only useful if you are 101% sure that nobody creates or deletes records "in between". However if you decide to try this method a good approach would be:

recordset = self.env['your.model'].search([('id', '!=', False), limit=1, order="id desc")

This will give you exactly one recordset (the newest one) & accordingly exactly one ID.

previous_id = recordset.ids[0]

So the "not yet existing" ID you are looking for is: newest_id = previous_id + 1

    @api.model
    def create(self, vals):
        res = super(SdProjectOverviewDiagram, self).create(vals)
        values = vals.get('values')[0][2]
        location_module = self.env['sd_project_overview.location']

        location_module.create({'diagram': res.id, 'value': 2})

        return res

This way you can create a record on another module while you are creating this record.

You can try:

@api.model
def create(self, vals):
    res = super(YourClassName, self).create(vals)
    print('Next ID will be:', res.id)
    return res

Thanks.