Fixed #29049 -- Added slicing notation to F expressions.

Co-authored-by: Priyansh Saxena <askpriyansh@gmail.com>
Co-authored-by: Niclas Olofsson <n@niclasolofsson.se>
Co-authored-by: David Smith <smithdc@gmail.com>
Co-authored-by: Mariusz Felisiak <felisiak.mariusz@gmail.com>
Co-authored-by: Abhinav Yadav <abhinav.sny.2002@gmail.com>

django/contrib/postgres/fields/array.py

@@ -234,6 +234,12 @@ class ArrayField(CheckFieldDefaultMixin, Field):
             }
         )
 
+    def slice_expression(self, expression, start, length):
+        # If length is not provided, don't specify an end to slice to the end
+        # of the array.
+        end = None if length is None else start + length - 1
+        return SliceTransform(start, end, expression)
+
 
 class ArrayRHSMixin:
     def __init__(self, lhs, rhs):
@@ -351,9 +357,11 @@ class SliceTransform(Transform):
 
     def as_sql(self, compiler, connection):
         lhs, params = compiler.compile(self.lhs)
-        if not lhs.endswith("]"):
-            lhs = "(%s)" % lhs
-        return "%s[%%s:%%s]" % lhs, (*params, self.start, self.end)
+        # self.start is set to 1 if slice start is not provided.
+        if self.end is None:
+            return f"({lhs})[%s:]", (*params, self.start)
+        else:
+            return f"({lhs})[%s:%s]", (*params, self.start, self.end)
 
 
 class SliceTransformFactory:

django/db/models/expressions.py

@@ -851,6 +851,9 @@ class F(Combinable):
     def __repr__(self):
         return "{}({})".format(self.__class__.__name__, self.name)
 
+    def __getitem__(self, subscript):
+        return Sliced(self, subscript)
+
     def resolve_expression(
         self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False
     ):
@@ -925,6 +928,63 @@ class OuterRef(F):
         return self
 
 
+class Sliced(F):
+    """
+    An object that contains a slice of an F expression.
+
+    Object resolves the column on which the slicing is applied, and then
+    applies the slicing if possible.
+    """
+
+    def __init__(self, obj, subscript):
+        super().__init__(obj.name)
+        self.obj = obj
+        if isinstance(subscript, int):
+            if subscript < 0:
+                raise ValueError("Negative indexing is not supported.")
+            self.start = subscript + 1
+            self.length = 1
+        elif isinstance(subscript, slice):
+            if (subscript.start is not None and subscript.start < 0) or (
+                subscript.stop is not None and subscript.stop < 0
+            ):
+                raise ValueError("Negative indexing is not supported.")
+            if subscript.step is not None:
+                raise ValueError("Step argument is not supported.")
+            if subscript.stop and subscript.start and subscript.stop < subscript.start:
+                raise ValueError("Slice stop must be greater than slice start.")
+            self.start = 1 if subscript.start is None else subscript.start + 1
+            if subscript.stop is None:
+                self.length = None
+            else:
+                self.length = subscript.stop - (subscript.start or 0)
+        else:
+            raise TypeError("Argument to slice must be either int or slice instance.")
+
+    def __repr__(self):
+        start = self.start - 1
+        stop = None if self.length is None else start + self.length
+        subscript = slice(start, stop)
+        return f"{self.__class__.__qualname__}({self.obj!r}, {subscript!r})"
+
+    def resolve_expression(
+        self,
+        query=None,
+        allow_joins=True,
+        reuse=None,
+        summarize=False,
+        for_save=False,
+    ):
+        resolved = query.resolve_ref(self.name, allow_joins, reuse, summarize)
+        if isinstance(self.obj, (OuterRef, self.__class__)):
+            expr = self.obj.resolve_expression(
+                query, allow_joins, reuse, summarize, for_save
+            )
+        else:
+            expr = resolved
+        return resolved.output_field.slice_expression(expr, self.start, self.length)
+
+
 @deconstructible(path="django.db.models.Func")
 class Func(SQLiteNumericMixin, Expression):
     """An SQL function call."""

django/db/models/fields/__init__.py

@@ -15,6 +15,7 @@ from django.core import checks, exceptions, validators
 from django.db import connection, connections, router
 from django.db.models.constants import LOOKUP_SEP
 from django.db.models.query_utils import DeferredAttribute, RegisterLookupMixin
+from django.db.utils import NotSupportedError
 from django.utils import timezone
 from django.utils.choices import (
     BlankChoiceIterator,
@@ -1143,6 +1144,10 @@ class Field(RegisterLookupMixin):
         """Return the value of this field in the given model instance."""
         return getattr(obj, self.attname)
 
+    def slice_expression(self, expression, start, length):
+        """Return a slice of this field."""
+        raise NotSupportedError("This field does not support slicing.")
+
 
 class BooleanField(Field):
     empty_strings_allowed = False
@@ -1303,6 +1308,11 @@ class CharField(Field):
             kwargs["db_collation"] = self.db_collation
         return name, path, args, kwargs
 
+    def slice_expression(self, expression, start, length):
+        from django.db.models.functions import Substr
+
+        return Substr(expression, start, length)
+
 
 class CommaSeparatedIntegerField(CharField):
     default_validators = [validators.validate_comma_separated_integer_list]
@@ -2497,6 +2507,11 @@ class TextField(Field):
             kwargs["db_collation"] = self.db_collation
         return name, path, args, kwargs
 
+    def slice_expression(self, expression, start, length):
+        from django.db.models.functions import Substr
+
+        return Substr(expression, start, length)
+
 
 class TimeField(DateTimeCheckMixin, Field):
     empty_strings_allowed = False