I have this issue when trying to remove a module (created for v6 but installed on v8, installed by a hired programmer) from Odoo.

Here is the error:

Traceback (most recent call last):

  File "/home/username/odoo/openerp/http.py", line 536, in _handle_exception

    return super(JsonRequest, self)._handle_exception(exception)

  File "/home/username/odoo/openerp/http.py", line 573, in dispatch

    result = self._call_function(**self.params)

  File "/home/username/odoo/openerp/http.py", line 309, in _call_function

    return checked_call(self.db, *args, **kwargs)

  File "/home/username/odoo/openerp/service/model.py", line 113, in wrapper

    return f(dbname, *args, **kwargs)

  File "/home/username/odoo/openerp/http.py", line 306, in checked_call

    return self.endpoint(*a, **kw)

  File "/home/username/odoo/openerp/http.py", line 802, in __call__

    return self.method(*args, **kw)

  File "/home/username/odoo/openerp/http.py", line 402, in response_wrap

    response = f(*args, **kw)

  File "/home/username/odoo/addons/web/controllers/main.py", line 941, in call_button

    action = self._call_kw(model, method, args, {})

  File "/home/username/odoo/addons/web/controllers/main.py", line 929, in _call_kw

    return getattr(request.registry.get(model), method)(request.cr, request.uid, *args, **kwargs)

  File "/home/username/odoo/openerp/api.py", line 241, in wrapper

    return old_api(self, *args, **kwargs)

  File "/home/username/odoo/openerp/addons/base/module/wizard/base_module_upgrade.py", line 105, in upgrade_module

    openerp.modules.registry.RegistryManager.new(cr.dbname, update_module=True)

  File "/home/username/odoo/openerp/modules/registry.py", line 370, in new

    openerp.modules.load_modules(registry._db, force_demo, status, update_module)

  File "/home/username/odoo/openerp/modules/loading.py", line 423, in load_modules

    registry['ir.module.module'].module_uninstall(cr, SUPERUSER_ID, modules_to_remove.values())

  File "/home/username/odoo/openerp/api.py", line 241, in wrapper

    return old_api(self, *args, **kwargs)

  File "/home/username/odoo/openerp/addons/base/module/module.py", line 462, in module_uninstall

    ir_model_data._module_data_uninstall(cr, uid, modules_to_remove, context)

  File "/home/username/odoo/openerp/api.py", line 241, in wrapper

    return old_api(self, *args, **kwargs)

  File "/home/username/odoo/openerp/addons/base/ir/ir_model.py", line 1217, in _module_data_uninstall

    ir_model_constraint._module_data_uninstall(cr, uid, constraint_ids, context)

  File "/home/username/odoo/openerp/api.py", line 241, in wrapper

    return old_api(self, *args, **kwargs)

  File "/home/username/odoo/openerp/addons/base/ir/ir_model.py", line 575, in _module_data_uninstall

    model_obj = self.pool[model]

  File "/home/username/odoo/openerp/modules/registry.py", line 102, in __getitem__

    return self.models[model_name]

KeyError: u'custom.purchase'

How can I remove this module?

Context: This was a module developed for us for v6 and it worked fine.  We installed v8 with a different programmer who I guess tried installing that v6 module which sent an email message alerting that a product had reached its Min Reorder Point.  Now we are unable to remove it.  

I found a post here:https://www.odoo.com/forum/help-1/question/error-when-uninstalling-a-custom-module-that-inherits-from-crm-lead-keyerror-crm-lead-74761

So we tried adding ['base', 'custom'] to depends: but that didnt help.

Any ideas?