refactor(core): Centralize core serializer components

 This commit centralizes several core serializer components into the `core` app to promote reusability and improve code organization across the project.

Key changes:
- **Moved Serializers to Core:** `BasePromoSerializer`, `TargetSerializer`, and `CountryField` have been moved from the `business` app to `core.serializers`. This allows them to be reused by other apps, such as `user`.
- **Simplified User Serializers:** The `UserFeedQuerySerializer` now inherits from the `BaseLimitOffsetPaginationSerializer`, and the `OtherFieldSerializer` now uses the
     centralized `core.serializers.CountryField`, reducing code duplication and simplifying validation.

Author: RandomProgramm3r

promo_code/business/serializers.py

@@ -2,7 +2,6 @@
 
 import django.contrib.auth.password_validation
 import django.db.transaction
-import pycountry
 import rest_framework.exceptions
 import rest_framework.serializers
 import rest_framework_simplejwt.exceptions
@@ -140,36 +139,14 @@ def get_active_company_from_token(self, token):
         return company
 
 
-class CountryField(rest_framework.serializers.CharField):
-    """
-    Custom field for validating country codes according to ISO 3166-1 alpha-2.
-    """
-
-    def __init__(self, **kwargs):
-        kwargs['allow_blank'] = False
-        kwargs['min_length'] = business.constants.TARGET_COUNTRY_CODE_LENGTH
-        kwargs['max_length'] = business.constants.TARGET_COUNTRY_CODE_LENGTH
-        super().__init__(**kwargs)
-
-    def to_internal_value(self, data):
-        code = super().to_internal_value(data)
-        try:
-            pycountry.countries.lookup(code.upper())
-        except LookupError:
-            raise rest_framework.serializers.ValidationError(
-                'Invalid ISO 3166-1 alpha-2 country code.',
-            )
-        return code
-
-
 class MultiCountryField(rest_framework.serializers.ListField):
     """
     Custom field for handling multiple country codes,
     passed either as a comma-separated list or as multiple parameters.
     """
 
     def __init__(self, **kwargs):
-        kwargs['child'] = CountryField()
+        kwargs['child'] = core.serializers.CountryField()
         kwargs['allow_empty'] = False
         super().__init__(**kwargs)
 
@@ -196,234 +173,14 @@ def to_internal_value(self, data):
         return super().to_internal_value(data)
 
 
-class TargetSerializer(rest_framework.serializers.Serializer):
-    age_from = rest_framework.serializers.IntegerField(
-        min_value=business.constants.TARGET_AGE_MIN,
-        max_value=business.constants.TARGET_AGE_MAX,
-        required=False,
-    )
-    age_until = rest_framework.serializers.IntegerField(
-        min_value=business.constants.TARGET_AGE_MIN,
-        max_value=business.constants.TARGET_AGE_MAX,
-        required=False,
-    )
-    country = CountryField(required=False)
-
-    categories = rest_framework.serializers.ListField(
-        child=rest_framework.serializers.CharField(
-            min_length=business.constants.TARGET_CATEGORY_MIN_LENGTH,
-            max_length=business.constants.TARGET_CATEGORY_MAX_LENGTH,
-            allow_blank=False,
-        ),
-        max_length=business.constants.TARGET_CATEGORY_MAX_ITEMS,
-        required=False,
-        allow_empty=True,
-    )
-
-    def validate(self, data):
-        age_from = data.get('age_from')
-        age_until = data.get('age_until')
-
-        if (
-            age_from is not None
-            and age_until is not None
-            and age_from > age_until
-        ):
-            raise rest_framework.serializers.ValidationError(
-                {'age_until': 'Must be greater than or equal to age_from.'},
-            )
-        return data
-
-
-class BasePromoSerializer(rest_framework.serializers.ModelSerializer):
-    """
-    Base serializer for promo, containing validation and representation logic.
-    """
-
-    image_url = rest_framework.serializers.URLField(
-        required=False,
-        allow_blank=False,
-        max_length=business.constants.PROMO_IMAGE_URL_MAX_LENGTH,
-    )
-    description = rest_framework.serializers.CharField(
-        min_length=business.constants.PROMO_DESC_MIN_LENGTH,
-        max_length=business.constants.PROMO_DESC_MAX_LENGTH,
-        required=True,
-    )
-    target = TargetSerializer(required=True, allow_null=True)
-    promo_common = rest_framework.serializers.CharField(
-        min_length=business.constants.PROMO_COMMON_CODE_MIN_LENGTH,
-        max_length=business.constants.PROMO_COMMON_CODE_MAX_LENGTH,
-        required=False,
-        allow_null=True,
-        allow_blank=False,
-    )
-    promo_unique = rest_framework.serializers.ListField(
-        child=rest_framework.serializers.CharField(
-            min_length=business.constants.PROMO_UNIQUE_CODE_MIN_LENGTH,
-            max_length=business.constants.PROMO_UNIQUE_CODE_MAX_LENGTH,
-            allow_blank=False,
-        ),
-        min_length=business.constants.PROMO_UNIQUE_LIST_MIN_ITEMS,
-        max_length=business.constants.PROMO_UNIQUE_LIST_MAX_ITEMS,
-        required=False,
-        allow_null=True,
-    )
-
-    class Meta:
-        model = business.models.Promo
-        fields = (
-            'description',
-            'image_url',
-            'target',
-            'max_count',
-            'active_from',
-            'active_until',
-            'mode',
-            'promo_common',
-            'promo_unique',
-        )
-
-    def validate(self, data):
-        """
-        Main validation method.
-        Determines the mode and calls the corresponding validation method.
-        """
-
-        mode = data.get('mode', getattr(self.instance, 'mode', None))
-
-        if mode == business.constants.PROMO_MODE_COMMON:
-            self._validate_common(data)
-        elif mode == business.constants.PROMO_MODE_UNIQUE:
-            self._validate_unique(data)
-        elif mode is None:
-            raise rest_framework.serializers.ValidationError(
-                {'mode': 'This field is required.'},
-            )
-        else:
-            raise rest_framework.serializers.ValidationError(
-                {'mode': 'Invalid mode.'},
-            )
-
-        return data
-
-    def _validate_common(self, data):
-        """
-        Validations for COMMON promo mode.
-        """
-
-        if 'promo_unique' in data and data['promo_unique'] is not None:
-            raise rest_framework.serializers.ValidationError(
-                {'promo_unique': 'This field is not allowed for COMMON mode.'},
-            )
-
-        if self.instance is None and not data.get('promo_common'):
-            raise rest_framework.serializers.ValidationError(
-                {'promo_common': 'This field is required for COMMON mode.'},
-            )
-
-        new_max_count = data.get('max_count')
-        if self.instance and new_max_count is not None:
-            used_count = self.instance.get_used_codes_count
-            if used_count > new_max_count:
-                raise rest_framework.serializers.ValidationError(
-                    {
-                        'max_count': (
-                            f'max_count ({new_max_count}) cannot be less than '
-                            f'used_count ({used_count}).'
-                        ),
-                    },
-                )
-
-        effective_max_count = (
-            new_max_count
-            if new_max_count is not None
-            else getattr(self.instance, 'max_count', None)
-        )
-
-        min_c = business.constants.PROMO_COMMON_MIN_COUNT
-        max_c = business.constants.PROMO_COMMON_MAX_COUNT
-        if effective_max_count is not None and not (
-            min_c <= effective_max_count <= max_c
-        ):
-            raise rest_framework.serializers.ValidationError(
-                {
-                    'max_count': (
-                        f'Must be between {min_c} and {max_c} for COMMON mode.'
-                    ),
-                },
-            )
-
-    def _validate_unique(self, data):
-        """
-        Validations for UNIQUE promo mode.
-        """
-
-        if 'promo_common' in data and data['promo_common'] is not None:
-            raise rest_framework.serializers.ValidationError(
-                {'promo_common': 'This field is not allowed for UNIQUE mode.'},
-            )
-
-        if self.instance is None and not data.get('promo_unique'):
-            raise rest_framework.serializers.ValidationError(
-                {'promo_unique': 'This field is required for UNIQUE mode.'},
-            )
-
-        effective_max_count = data.get(
-            'max_count',
-            getattr(self.instance, 'max_count', None),
-        )
-
-        if (
-            effective_max_count is not None
-            and effective_max_count
-            != business.constants.PROMO_UNIQUE_MAX_COUNT
-        ):
-            raise rest_framework.serializers.ValidationError(
-                {
-                    'max_count': (
-                        'Must be equal to '
-                        f'{business.constants.PROMO_UNIQUE_MAX_COUNT} '
-                        'for UNIQUE mode.'
-                    ),
-                },
-            )
-
-    def to_representation(self, instance):
-        """
-        Controls the display of fields in the response.
-        """
-
-        data = super().to_representation(instance)
-
-        if not instance.image_url:
-            data.pop('image_url', None)
-
-        if instance.mode == business.constants.PROMO_MODE_UNIQUE:
-            data.pop('promo_common', None)
-            if 'promo_unique' in self.fields and isinstance(
-                self.fields['promo_unique'],
-                rest_framework.serializers.SerializerMethodField,
-            ):
-                data['promo_unique'] = self.get_promo_unique(instance)
-            else:
-                data['promo_unique'] = [
-                    code.code for code in instance.unique_codes.all()
-                ]
-        else:
-            data.pop('promo_unique', None)
-
-        return data
-
-
-class PromoCreateSerializer(BasePromoSerializer):
+class PromoCreateSerializer(core.serializers.BasePromoSerializer):
     url = rest_framework.serializers.HyperlinkedIdentityField(
         view_name='api-business:promo-detail',
         lookup_field='id',
     )
 
-    class Meta(BasePromoSerializer.Meta):
-        fields = ('url',) + BasePromoSerializer.Meta.fields
+    class Meta(core.serializers.BasePromoSerializer.Meta):
+        fields = ('url',) + core.serializers.BasePromoSerializer.Meta.fields
 
     def create(self, validated_data):
         target_data = validated_data.pop('target')
@@ -468,7 +225,7 @@ def validate(self, attrs):
         return attrs
 
 
-class PromoDetailSerializer(BasePromoSerializer):
+class PromoDetailSerializer(core.serializers.BasePromoSerializer):
     promo_id = rest_framework.serializers.UUIDField(
         source='id',
         read_only=True,
@@ -496,8 +253,8 @@ class PromoDetailSerializer(BasePromoSerializer):
 
     promo_unique = rest_framework.serializers.SerializerMethodField()
 
-    class Meta(BasePromoSerializer.Meta):
-        fields = BasePromoSerializer.Meta.fields + (
+    class Meta(core.serializers.BasePromoSerializer.Meta):
+        fields = core.serializers.BasePromoSerializer.Meta.fields + (
             'promo_id',
             'company_name',
             'like_count',

promo_code/core/pagination.py

@@ -1,4 +1,3 @@
-import rest_framework.exceptions
 import rest_framework.pagination
 import rest_framework.response