pos_usability: add group by on pos category on products search view

Add link to products from pos.category form view (same as on product.category form view)

pos_usability/pos.py

@@ -0,0 +1,25 @@
+# -*- coding: utf-8 -*-
+# © 2017 Akretion (Alexis de Lattre <alexis.delattre@akretion.com>)
+# License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl).
+
+from odoo import fields, models
+
+
+class PosCategory(models.Model):
+    _inherit = 'pos.category'
+
+    product_count = fields.Integer(
+        '# Products', compute='_compute_product_count',
+        help="The number of products under this point of sale category "
+        "(does not consider the children categories)")
+
+    # inspired by the code of odoo/addons/product/models/product.py
+    def _compute_product_count(self):
+        read_group_res = self.env['product.template'].read_group(
+            [('pos_categ_id', 'in', self.ids)],
+            ['pos_categ_id'], ['pos_categ_id'])
+        group_data = dict(
+            (data['pos_categ_id'][0], data['pos_categ_id_count']) for data
+            in read_group_res)
+        for pos_categ in self:
+            pos_categ.product_count = group_data.get(pos_categ.id, 0)