Hi,

Ensure that:

1. user_id in mymodel.history is defined as a Many2one field linking to res.users .
2. The field is displayed in views (form/tree) with the correct relation and context.

Model Definition:

from odoo import models, fields

class  MyModelHistory (models.Model):
    _name = 'mymodel.history' 
    _description = 'My Model History'

    user_id = fields.Many2one(
        'res.users' ,
        string = 'User' ,
        default= lambda self: self.env.uid
    )
    mymodel_id = fields.Many2one( 'mymodel' , string= 'My Model' )
    old_state = fields.Char( 'Old State' )
    new_state = fields.Char( 'New State' )

View Definition Example (XML):

<record id = "view_mymodel_history_tree"  model = "ir.ui.view" >
     <field  name = "name" >mymodel.history.tree </field>
     <field  name = "model" >mymodel.history </field>
     <field  name = "arch"  type = "xml">
         <tree>
             <field name = "user_id"/>
             <field name = "mymodel_id"/>
             <field  name = "old_state"/>
             <field  name = "new_state"/>
         </tree>
     </field>
 </record>

Creating Records in Code:

What you have is fine, but for clarity, you can use:

def  _create_history_record ( self, old_state, new_state ):
     for rec in self:
        self.env[ "mymodel.history" ].create({
             "user_id" : self.env.uid,
             "mymodel_id" : rec. id ,
             "old_state" : old_state,
             "new_state" : new_state,
        })

or even better (Odoo best practice):

def  _create_history_record ( self, old_state, new_state ):
     for rec in self:
        self.env[ "mymodel.history" ].sudo().create({
             "user_id" : self.env.user. id ,
             "mymodel_id" : rec. id ,
             "old_state" : old_state,
             "new_state" : new_state,
        })

i hope it is usefull