Fix root model reuse collapse

src/datamodel_code_generator/parser/base.py

@@ -1815,6 +1815,8 @@ def __reuse_model(self, models: list[DataModel], require_update_action_models: l
         model_cache: dict[tuple[HashableComparable, ...], Reference] = {}
         duplicates = []
         for model in models.copy():
+            if self.collapse_root_models and isinstance(model, self.data_model_root_type):
+                continue
             model_key = model.get_dedup_key()
             cached_model_reference = model_cache.get(model_key)
             if cached_model_reference:

tests/data/expected/main/jsonschema/reuse_model_collapse_root_models_constraints.py

@@ -0,0 +1,30 @@
+# generated by datamodel-codegen:
+#   filename:  reuse_model_collapse_root_models_constraints.json
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from pydantic import BaseModel, Field, RootModel
+
+
+class Shared(BaseModel):
+    short_code: str = Field(
+        ..., alias='shortCode', max_length=8, min_length=0, title='ShortCode'
+    )
+
+
+class RecordA(Shared):
+    timestamp: str = Field(..., title='Timestamp')
+
+
+class RecordB(Shared):
+    created: str = Field(..., title='Timestamp')
+    label: str = Field(..., title='LongText')
+
+
+class ReproBugs(RootModel[RecordA | RecordB]):
+    root: RecordA | RecordB = Field(
+        ...,
+        description='Minimal schema to reproduce reuse_model + collapse_root_models bugs in datamodel-code-generator 0.55.0',
+        title='ReproBugs',
+    )

tests/data/jsonschema/reuse_model_collapse_root_models_constraints.json

@@ -0,0 +1,77 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "ReproBugs",
+  "description": "Minimal schema to reproduce reuse_model + collapse_root_models bugs in datamodel-code-generator 0.55.0",
+  "$defs": {
+    "Shared": {
+      "type": "object",
+      "title": "Shared",
+      "properties": {
+        "shortCode": {
+          "title": "ShortCode",
+          "type": "string",
+          "maxLength": 8,
+          "minLength": 0
+        }
+      },
+      "required": [
+        "shortCode"
+      ]
+    },
+    "RecordA": {
+      "type": "object",
+      "title": "RecordA",
+      "allOf": [
+        {
+          "$ref": "#/$defs/Shared"
+        },
+        {
+          "type": "object",
+          "properties": {
+            "timestamp": {
+              "title": "Timestamp",
+              "type": "string"
+            }
+          },
+          "required": [
+            "timestamp"
+          ]
+        }
+      ]
+    },
+    "RecordB": {
+      "type": "object",
+      "title": "RecordB",
+      "allOf": [
+        {
+          "$ref": "#/$defs/Shared"
+        },
+        {
+          "type": "object",
+          "properties": {
+            "created": {
+              "title": "Timestamp",
+              "type": "string"
+            },
+            "label": {
+              "title": "LongText",
+              "type": "string"
+            }
+          },
+          "required": [
+            "created",
+            "label"
+          ]
+        }
+      ]
+    }
+  },
+  "oneOf": [
+    {
+      "$ref": "#/$defs/RecordA"
+    },
+    {
+      "$ref": "#/$defs/RecordB"
+    }
+  ]
+}

tests/main/jsonschema/test_main_jsonschema.py

@@ -1338,6 +1338,37 @@ def test_main_reuse_model_collapse_nested(output_file: Path) -> None:
     )
 
 
+@pytest.mark.parametrize(
+    "collapse_reuse_model_args",
+    [[], ["--collapse-reuse-models"]],
+)
+def test_main_reuse_model_collapse_root_models_preserves_constraints(
+    output_file: Path, collapse_reuse_model_args: list[str]
+) -> None:
+    """Test root-model collapse keeps distinct field constraints when titles collide."""
+    extra_args = [
+        "--reuse-model",
+        "--collapse-root-models",
+        "--use-title-as-name",
+        "--field-constraints",
+        "--snake-case-field",
+        "--target-python-version",
+        "3.10",
+        "--output-model-type",
+        "pydantic_v2.BaseModel",
+        *collapse_reuse_model_args,
+    ]
+
+    run_main_and_assert(
+        input_path=JSON_SCHEMA_DATA_PATH / "reuse_model_collapse_root_models_constraints.json",
+        output_path=output_file,
+        input_file_type="jsonschema",
+        assert_func=assert_file_content,
+        expected_file="reuse_model_collapse_root_models_constraints.py",
+        extra_args=extra_args,
+    )
+
+
 @pytest.mark.cli_doc(
     options=["--capitalize-enum-members"],
     option_description="""Capitalize enum member names to UPPER_CASE format.