Tweak ordering of definitions in generated schemas

pydantic/_internal/_generate_schema.py

@@ -437,6 +437,8 @@ def clean_schema(self, schema: CoreSchema) -> CoreSchema:
         if collect_invalid_schemas(schema):
             raise self.CollectedInvalid()
         schema = validate_core_schema(schema)
+        if 'definitions' in schema:
+            schema['definitions'] = list(reversed(schema['definitions']))
         return schema
 
     def collect_definitions(self, schema: CoreSchema) -> CoreSchema:

tests/test_discriminated_union.py

@@ -2,7 +2,7 @@
 import sys
 from enum import Enum, IntEnum
 from types import SimpleNamespace
-from typing import Any, Callable, Generic, Optional, Sequence, TypeVar, Union
+from typing import Any, Callable, Generic, List, Optional, Sequence, TypeVar, Union
 
 import pytest
 from dirty_equals import HasRepr, IsStr
@@ -12,6 +12,8 @@
 from pydantic import BaseModel, ConfigDict, Discriminator, Field, TypeAdapter, ValidationError, field_validator
 from pydantic._internal._discriminated_union import apply_discriminator
 from pydantic.errors import PydanticUserError
+from pydantic.fields import FieldInfo
+from pydantic.json_schema import GenerateJsonSchema
 from pydantic.types import Tag
 
 
@@ -1696,3 +1698,68 @@ class DiscriminatedModel(BaseModel):
             ]
     except PydanticUserError as exc_info:
         assert exc_info.code == 'callable-discriminator-no-tag'
+
+
+def test_presence_of_discriminator_when_generating_type_adaptor_json_schema_definitions() -> None:
+    class ItemType(str, Enum):
+        ITEM1 = 'item1'
+        ITEM2 = 'item2'
+
+    class CreateItem1(BaseModel):
+        item_type: Annotated[Literal[ItemType.ITEM1], Field(alias='type')]
+        id: int
+
+    class CreateItem2(BaseModel):
+        item_type: Annotated[Literal[ItemType.ITEM2], Field(alias='type')]
+        id: int
+
+    class CreateObjectDto(BaseModel):
+        id: int
+        items: List[
+            Annotated[
+                Union[
+                    CreateItem1,
+                    CreateItem2,
+                ],
+                Field(discriminator='item_type'),
+            ]
+        ]
+
+    adaptor = TypeAdapter(
+        Annotated[CreateObjectDto, FieldInfo(examples=[{'id': 1, 'items': [{'id': 3, 'type': 'ITEM1'}]}])]
+    )
+
+    schema_map, definitions = GenerateJsonSchema().generate_definitions([(adaptor, 'validation', adaptor.core_schema)])
+    assert definitions == {
+        'CreateItem1': {
+            'properties': {'id': {'title': 'Id', 'type': 'integer'}, 'type': {'const': 'item1', 'title': 'Type'}},
+            'required': ['type', 'id'],
+            'title': 'CreateItem1',
+            'type': 'object',
+        },
+        'CreateItem2': {
+            'properties': {'id': {'title': 'Id', 'type': 'integer'}, 'type': {'const': 'item2', 'title': 'Type'}},
+            'required': ['type', 'id'],
+            'title': 'CreateItem2',
+            'type': 'object',
+        },
+        'CreateObjectDto': {
+            'properties': {
+                'id': {'title': 'Id', 'type': 'integer'},
+                'items': {
+                    'items': {
+                        'discriminator': {
+                            'mapping': {'item1': '#/$defs/CreateItem1', 'item2': '#/$defs/CreateItem2'},
+                            'propertyName': 'type',
+                        },
+                        'oneOf': [{'$ref': '#/$defs/CreateItem1'}, {'$ref': '#/$defs/CreateItem2'}],
+                    },
+                    'title': 'Items',
+                    'type': 'array',
+                },
+            },
+            'required': ['id', 'items'],
+            'title': 'CreateObjectDto',
+            'type': 'object',
+        },
+    }