<filename>myproject/sales/admin.py
from django import forms
from ra.admin.admin import ra_admin_site, RaAdmin, RaMovementAdmin
from .models import Client, Product, SimpleSales


class ClientAdmin(RaAdmin):
    fields = ('slug', 'title', 'notes', 'address', 'email', 'telephone')
    view_template = 'sales/admin/client_view.html'



class ProductAdmin(RaAdmin):
    pass


class SalesOrderAdmin(RaMovementAdmin):
    fields = ['slug', 'doc_date', 'client', ('product', 'price', 'quantity', 'value')]
    add_form_template = change_form_template = 'sales/admin/salesorder_changeform.html'

    def formfield_for_dbfield(self, db_field, request, **kwargs):
        formfield = super().formfield_for_dbfield(db_field, request, **kwargs)
        if db_field.name == 'value':
            formfield.widget = forms.TextInput(attrs={'readonly': 'readonly'})
        return formfield


ra_admin_site.register(Client, ClientAdmin)
ra_admin_site.register(Product, ProductAdmin)
ra_admin_site.register(SimpleSales, SalesOrderAdmin)

<filename>myproject/sales/admin.py
from django import forms
from ra.admin.admin import ra_admin_site, RaAdmin, RaMovementAdmin
from .models import Client, Product, SimpleSales


class ClientAdmin(RaAdmin):
    fields = ('slug', 'title', 'notes', 'address', 'email', 'telephone')
    view_template = 'sales/admin/client_view.html'



class ProductAdmin(RaAdmin):
    pass


class SalesOrderAdmin(RaMovementAdmin):
    fields = ['slug', 'doc_date', 'client', ('product', 'price', 'quantity', 'value')]
    add_form_template = change_form_template = 'sales/admin/salesorder_changeform.html'

    def formfield_for_dbfield(self, db_field, request, **kwargs):
        formfield = super().formfield_for_dbfield(db_field, request, **kwargs)
        if db_field.name == 'value':
            formfield.widget = forms.TextInput(attrs={'readonly': 'readonly'})
        return formfield


ra_admin_site.register(Client, ClientAdmin)
ra_admin_site.register(Product, ProductAdmin)
ra_admin_site.register(SimpleSales, SalesOrderAdmin)
