I looked at different forum posts but I don't see a fix. I have 2 custom models that are in a relation One2many. One the many side I have a few related fields just for displaying the data in the view. Now if I change a value on the One side model that is related to the many side model I get the Expected singleton: my_custom_model_many(id1, id2, id3).

How can I fix this? I tough to make create and  write methods @api.multi and then iterate with "for rec in self:" but they have return statements so the methods would finish in the first iteration. 

If anyone has any suggestions with maybe a code example, that would be very appreciated!  

class my_custom_module_one(models.Model):
	_name = 'my.custom.one'
	month = fields.Char()
	date_from = fields.Date()
	date_to = fields.Date()

	many_ids = fields.One2many('my.custom.many', 'one_id')

class my_custom_module_many(models.Model):
	_name = 'my.custom.many'
	one_month = fields.Char(related='one_id.month')
	one_date_form = fields.Date(related='one_id.date_from')
	one_date_to = fields.Date(related='one_id.date_to')

	one_id = fields.Many2one('my.custom.one')