#31640: Trunc() function take tzinfo param into account only when 
DateTimeField()
are used as output_field
-------------------------------------+-------------------------------------
     Reporter:  Serhii Romanov       |                    Owner:  nobody
         Type:  Bug                  |                   Status:  new
    Component:  Database layer       |                  Version:  master
  (models, ORM)                      |
     Severity:  Normal               |               Resolution:
     Keywords:  trunc timezone tz    |             Triage Stage:  Accepted
    Has patch:  0                    |      Needs documentation:  0
  Needs tests:  0                    |  Patch needs improvement:  0
Easy pickings:  0                    |                    UI/UX:  0
-------------------------------------+-------------------------------------
Changes (by Simon Charette):

 * stage:  Unreviewed => Accepted


Comment:

 It looks like the logic should be based off `self.lhs.output_field`
 instead.

 Assuming you are using PostgreSQL does the following patch addresses your
 issue?

 {{{#!diff
 diff --git a/django/db/backends/postgresql/operations.py
 b/django/db/backends/postgresql/operations.py
 index c67062a4a7..3877a38b43 100644
 --- a/django/db/backends/postgresql/operations.py
 +++ b/django/db/backends/postgresql/operations.py
 @@ -38,10 +38,6 @@ class DatabaseOperations(BaseDatabaseOperations):
          else:
              return "EXTRACT('%s' FROM %s)" % (lookup_type, field_name)

 -    def date_trunc_sql(self, lookup_type, field_name):
 -        # https://www.postgresql.org/docs/current/functions-datetime.html
 #FUNCTIONS-DATETIME-TRUNC
 -        return "DATE_TRUNC('%s', %s)" % (lookup_type, field_name)
 -
      def _prepare_tzname_delta(self, tzname):
          if '+' in tzname:
              return tzname.replace('+', '-')
 @@ -50,7 +46,7 @@ class DatabaseOperations(BaseDatabaseOperations):
          return tzname

      def _convert_field_to_tz(self, field_name, tzname):
 -        if settings.USE_TZ:
 +        if tzname and settings.USE_TZ:
              field_name = "%s AT TIME ZONE '%s'" % (field_name,
 self._prepare_tzname_delta(tzname))
          return field_name

 @@ -71,7 +67,16 @@ class DatabaseOperations(BaseDatabaseOperations):
          # https://www.postgresql.org/docs/current/functions-datetime.html
 #FUNCTIONS-DATETIME-TRUNC
          return "DATE_TRUNC('%s', %s)" % (lookup_type, field_name)

 -    def time_trunc_sql(self, lookup_type, field_name):
 +    def date_trunc_sql(self, lookup_type, field_name, tzname=None):
 +        if tzname:
 +            field_name = self._convert_field_to_tz(field_name, tzname)
 +        # https://www.postgresql.org/docs/current/functions-datetime.html
 #FUNCTIONS-DATETIME-TRUNC
 +        return "DATE_TRUNC('%s', %s)" % (lookup_type, field_name)
 +
 +    def time_trunc_sql(self, lookup_type, field_name, tzname=None):
 +        if tzname:
 +            field_name = self._convert_field_to_tz(field_name, tzname)
 +        field_name = self._convert_field_to_tz(field_name, tzname)
          return "DATE_TRUNC('%s', %s)::time" % (lookup_type, field_name)

      def json_cast_text_sql(self, field_name):
 diff --git a/django/db/models/functions/datetime.py
 b/django/db/models/functions/datetime.py
 index b6594b043b..52714aa9bf 100644
 --- a/django/db/models/functions/datetime.py
 +++ b/django/db/models/functions/datetime.py
 @@ -191,13 +191,15 @@ class TruncBase(TimezoneMixin, Transform):

      def as_sql(self, compiler, connection):
          inner_sql, inner_params = compiler.compile(self.lhs)
 -        if isinstance(self.output_field, DateTimeField):
 +        tzname = None
 +        if isinstance(self.lhs.output_field, DateTimeField):
              tzname = self.get_tzname()
 +        if isinstance(self.output_field, DateTimeField):
              sql = connection.ops.datetime_trunc_sql(self.kind, inner_sql,
 tzname)
          elif isinstance(self.output_field, DateField):
 -            sql = connection.ops.date_trunc_sql(self.kind, inner_sql)
 +            sql = connection.ops.date_trunc_sql(self.kind, inner_sql,
 tzname)
          elif isinstance(self.output_field, TimeField):
 -            sql = connection.ops.time_trunc_sql(self.kind, inner_sql)
 +            sql = connection.ops.time_trunc_sql(self.kind, inner_sql,
 tzname)
          else:
              raise ValueError('Trunc only valid on DateField, TimeField,
 or DateTimeField.')
          return sql, inner_params
 }}}

-- 
Ticket URL: <https://code.djangoproject.com/ticket/31640#comment:1>
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