[permission] Add F object support

apps/permission/models.py

@@ -5,7 +5,7 @@ import operator
 from django.contrib.contenttypes.models import ContentType
 from django.core.exceptions import ValidationError
 from django.db import models
-from django.db.models import Q
+from django.db.models import F, Q
 from django.utils.translation import gettext_lazy as _
 
 
@@ -58,13 +58,20 @@ class Permission(models.Model):
 
     # A json encoded Q object with the following grammar
     #  query -> [] | {}  (the empty query representing all objects)
-    #  query -> ['AND', query, …]
-    #        -> ['OR', query, …]
-    #        -> ['NOT', query]
-    #  query -> {key: value, …}
-    #  key   -> string
-    #  value -> int | string | bool | null
-    #        -> [parameter]
+    #  query -> ['AND', query, …]            AND multiple queries
+    #         | ['OR', query, …]             OR multiple queries
+    #         | ['NOT', query]               Opposite of query
+    #  query -> {key: value, …}              A list of fields and values of a Q object
+    #  key   -> string                       A field name
+    #  value -> int | string | bool | null   Literal values
+    #         | [parameter]                  A parameter
+    #         | {'F': oper}                  An F object
+    #  oper  -> [string]                     A parameter
+    #         | ['ADD', oper, …]             Sum multiple F objects or literal
+    #         | ['SUB', oper, oper]          Substract two F objects or literal
+    #         | ['MUL', oper, …]             Multiply F objects or literals
+    #         | int | string | bool | null   Literal values
+    #         | ['F', string]                A field
     #
     # Examples:
     #  Q(is_admin=True)  := {'is_admin': ['TYPE', 'bool', 'True']}
@@ -88,6 +95,26 @@ class Permission(models.Model):
         self.full_clean()
         super().save()
 
+    @staticmethod
+    def compute_f(_oper, **kwargs):
+        oper = _oper
+        if isinstance(oper, list):
+            if len(oper) == 1:
+                return kwargs[oper[0]].pk
+            elif len(oper) >= 2:
+                if oper[0] == 'ADD':
+                    return functools.reduce(operator.add, [compute_f(oper, **kwargs) for oper in oper[1:]])
+                elif oper[0] == 'SUB':
+                    return compute_f(oper[1], **kwargs) - compute_f(oper[2], **kwargs)
+                elif oper[0] == 'MUL':
+                    return functools.reduce(operator.mul, [compute_f(oper, **kwargs) for oper in oper[1:]])
+                elif oper[0] == 'F':
+                    return F(oper[1])
+        else:
+            return oper
+        # TODO: find a better way to crash here
+        raise Exception("F is wrong")
+
     def _about(_self, _query, **kwargs):
         self = _self
         query = _query
@@ -110,6 +137,9 @@ class Permission(models.Model):
                 if isinstance(value, list):
                     # It is a parameter we query its primary key
                     q_kwargs[key] = kwargs[value[0]].pk
+                elif isinstance(value, dict):
+                    # It is an F object
+                    q_kwargs[key] = compute_f(query['F'], **kwargs)
                 else:
                     q_kwargs[key] = value
             return Q(**q_kwargs)