[ADD] contract_variable_quantity:

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Variable quantity in contract recurrent invoicing
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With this module, you will be able to define in recurring contracts some
lines with variable quantity according a provided formula.

Configuration
=============

* Go to Sales > Configuration > Contracts > Formulas (quantity).
* Define any formula based on Python code that stores at some moment a
  float/integer value of the quantity to invoice in the variable 'result'.

  You can use these variables to compute your formula:

  * *env*: Environment variable for getting other models.
  * *context*: Current context dictionary.
  * *user*: Current user.
  * *line*: Contract recurring invoice line that triggers this formula.
  * *contract*: Contract whose line belongs to.
  * *invoice*: Invoice (header) being created.

Usage
=====

To use this module, you need to:

* Go to Sales -> Contracts and select or create a new contract.
* Check *Generate recurring invoices automatically*.
* Add a new recurring invoicing line.
* Select "Variable quantity" in column "Qty. type".
* Select one of the possible formulas to use (previously created).

contract_variable_quantity/models/contract.py

@@ -0,0 +1,66 @@
+# -*- coding: utf-8 -*-
+# © 2016 Pedro M. Baeza <pedro.baeza@tecnativa.com>
+# License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl).
+
+from openerp import _, api, fields, models, exceptions
+from openerp.tools.safe_eval import safe_eval
+
+
+class AccountAnalyticAccount(models.Model):
+    _inherit = "account.analytic.account"
+
+    @api.model
+    def _prepare_invoice_line(self, line, invoice_id):
+        vals = super(AccountAnalyticAccount, self)._prepare_invoice_line(
+            line, invoice_id)
+        if line.qty_type == 'variable':
+            eval_context = {
+                'env': self.env,
+                'context': self.env.context,
+                'user': self.env.user,
+                'line': line,
+                'contract': line.analytic_account_id,
+                'invoice': self.env['account.invoice'].browse(invoice_id),
+            }
+            safe_eval(line.qty_formula_id.code.strip(), eval_context,
+                      mode="exec", nocopy=True)  # nocopy for returning result
+            vals['quantity'] = eval_context.get('result', 0)
+        return vals
+
+
+class AccountAnalyticInvoiceLine(models.Model):
+    _inherit = 'account.analytic.invoice.line'
+
+    qty_type = fields.Selection(
+        selection=[
+            ('fixed', 'Fixed quantity'),
+            ('variable', 'Variable quantity'),
+        ], required=True, default='fixed', string="Qty. type")
+    qty_formula_id = fields.Many2one(
+        comodel_name="contract.line.qty.formula", string="Qty. formula")
+
+
+class ContractLineFormula(models.Model):
+    _name = 'contract.line.qty.formula'
+
+    name = fields.Char(required=True)
+    code = fields.Text(required=True, default="result = 0")
+
+    @api.constrains('code')
+    def _check_code(self):
+        eval_context = {
+            'env': self.env,
+            'context': self.env.context,
+            'user': self.env.user,
+            'line': self.env['account.analytic.invoice.line'],
+            'contract': self.env['account.analytic.account'],
+            'invoice': self.env['account.invoice'],
+        }
+        try:
+            safe_eval(
+                self.code.strip(), eval_context, mode="exec", nocopy=True)
+        except Exception as e:
+            raise exceptions.ValidationError(
+                _('Error evaluating code.\nDetails: %s') % e)
+        if 'result' not in eval_context:
+            raise exceptions.ValidationError(_('No valid result returned.'))