Read default values from database when no config is provided

Automatically add <field_name>_env_default for every field transformed
to a "computed from env" field, so a default value can be set. It will
be used when the configuration is not set in a configuration file
(when the key is absent, not empty).

server_environment/__manifest__.py

@@ -20,8 +20,10 @@
 
 {
     "name": "server configuration environment files",
-    "version": "11.0.1.0.1",
-    "depends": ["base"],
+    "depends": [
+        "base",
+        "base_sparse_field",
+    ],
     "author": "Camptocamp,Odoo Community Association (OCA)",
     "summary": "move some configurations out of the database",
     "website": "http://odoo-community.org/",

server_environment/models/server_env_mixin.py

@@ -3,7 +3,7 @@
 
 import logging
 
-from odoo import api, models
+from odoo import api, fields, models
 from ..serv_config import serv_config
 
 _logger = logging.getLogger(__name__)
@@ -22,19 +22,27 @@ class ServerEnvMixin(models.AbstractModel):
             def _server_env_fields(self):
                 return {"directory_path": 'get'}
 
-    With the snippet above, the "storage.backend" model will now use a server
+    With the snippet above, the "storage.backend" model now uses a server
     environment configuration for the field ``directory_path``.
 
-    Under the hood, this mixin will automatically replaces the original field
+    Under the hood, this mixin automatically replaces the original field
     by a computed field that reads from the configuration files.
 
-    By default, it will look for the configuration in a section named
+    By default, it looks for the configuration in a section named
     ``[model_name.Record Name]`` where ``model_name`` is the ``_name`` of the
     model with ``.`` replaced by ``_``. It can be customized by overriding the
     method :meth:`~server_env_section_name`.
+
+    For each field transformed to an env-computed field, a companion field
+    ``<field>_env_default`` is automatically created. When it's value is set
+    and the configuration files do not contain a key, the env-computed field
+    uses the default value stored in database. If a key is empty, the
+    env-computed field has an empty value.
     """
     _name = 'server.env.mixin'
 
+    server_env_defaults = fields.Serialized()
+
     @property
     def _server_env_fields(self):
         """Dict of fields to replace by fields computed from env
@@ -92,11 +100,24 @@ class ServerEnvMixin(models.AbstractModel):
         for record in self:
             for field_name, getter_name in self._server_env_fields.items():
                 section_name = self._server_env_section_name()
-                value = self._server_env_read_from_config(
-                    section_name, field_name, getter_name
-                )
+                if (section_name in serv_config
+                        and field_name in serv_config[section_name]):
+
+                    value = self._server_env_read_from_config(
+                        section_name, field_name, getter_name
+                    )
+
+                else:
+                    default_field = self._server_env_default_fieldname(
+                        field_name
+                    )
+                    value = record[default_field]
+
                 record[field_name] = value
 
+    def _server_env_default_fieldname(self, base_field_name):
+        return '%s_env_default' % (base_field_name,)
+
     def _server_env_transform_field_to_read_from_env(self, field):
         """Transform the original field in a computed field"""
         field.compute = '_compute_server_env'
@@ -104,9 +125,23 @@ class ServerEnvMixin(models.AbstractModel):
         field.copy = False
         field.sparse = None
 
+    def _server_env_add_default_field(self, base_field):
+        fieldname = self._server_env_default_fieldname(base_field.name)
+        if fieldname not in self._fields:
+            base_field_cls = base_field.__class__
+            field_args = base_field.args
+            field_args.pop('_sequence', None)
+            field_args.update({
+                'sparse': 'server_env_defaults',
+                'automatic': True,
+            })
+            field = base_field_cls(**field_args)
+            self._add_field(fieldname, field)
+
     @api.model
     def _setup_base(self):
         super()._setup_base()
         for fieldname in self._server_env_fields:
             field = self._fields[fieldname]
             self._server_env_transform_field_to_read_from_env(field)
+            self._server_env_add_default_field(field)