Describe the bug
I have used datamodel-code-generator with a custom mapping to create a Pydantic v2 model. Doing that resulted in field names having the same name as other models which messes up validation for Optional fields.
I believe the bug (?) is, independent of specific settings, present for all cases where one might use optional fields and a mapping.

To Reproduce
The JSON Schema roughly looks like this

{
  "title": "title1",
  "type": "object",
  "properties": {
    "F00000001173": {
      "title": "title2",
      "type": "string",
      "enum": ["001", "002", "003"]
    },
    "G00000001490": {
      "title": "title3",
      "type": "object",
      "properties": {
        "G00000001477": {
          "title": "title4",
          "type": "object",
          "properties": {},
          "required": ["F60000000227", "G60000000083", "G60000000093"]
        },
        "F00000002548": {
          "title": "title5",
          "type": "number",
          "multipleOf": 0.01,
          "minimum": 0
        },
        "F60000000296": {
          "type": "array",
          "items": {
            "title": "title6",
            "type": "string"
          },
          "minItems": 0
        }
      },
      "required": ["G00000001477", "F00000002548", "F60000000296"]
    }
  },
  "required": ["F00000001173"]
}

and a mapping between the ID fields and some more descriptive names was used (to make them more readable).

{
  "F00000002548": "field1",
  "G00000001490": "Model2",
  "F00000001173": "field3",
  "F60000000296": "field4"
}

This resulted in a code that looks like this

class Model2(BaseModel):
    model_config = ConfigDict(
        extra="allow",
        populate_by_name=True,
    )
    field1: Annotated[float, Field(alias="F00000002548", ge=0.0, multiple_of=0.01)]
    field2: Annotated[List[str], Field(alias="F60000000296", min_length=0)]


class Model1(BaseModel):
    model_config = ConfigDict(
        extra="allow",
        populate_by_name=True,
    )
    field1: Annotated[Literal["001", "002", "003"], Field(alias="F00000001173")]
    Model2: Annotated[Optional[Model2], Field(alias="G00000001490")] = None

where the problem is that the field should not be called Model2 like the class is.

Used commandline:

$ datamodel-codegen --input schema.json --target-python-version 3.10 --use-annotated --field-constraints --enum-field-as-literal all --allow-population-by-field-name --allow-population-by-field-name --output-model-type pydantic_v2.BaseModel --output schema.py

Expected behavior
The issue is fixed by making sure that the field names aren't the same as the other model names.
I achieved this temporarily by modifying the .jinja2 template like this

{%- for field in fields -%}
    {%- if not field.annotated and field.field %}
    {{ field.name | lower }}: {{ field.type_hint }} = {{ field.field }}
    {%- else %}
    {%- if field.annotated %}
    {{ field.name | lower }}: {{ field.annotated }}
    {%- else %}
    {{ field.name | lower }}: {{ field.type_hint }}
    {%- endif %}
    {%- if not field.required or field.data_type.is_optional or field.nullable
            %} = {{ field.represented_default }}
    {%- endif -%}
    {%- endif %}
    {%- if field.docstring %}
    """
    {{ field.docstring | indent(4) }}
    """
    {%- endif %}

Version:

• OS: [e.g. iOS]
• Python version: 3.10
• datamodel-code-generator version: 0.22.0

Additional context
Is there a way to modify this consistently? Passing the custom_template_dir argument didn't seem to work for me.
If this is indeed a bug, I'm happy to provide a PR if someone can tell me where the field names are created! :)