Add relationship() with back_populates for SQLAlchemy models (issue #47)

CHANGELOG.md

@@ -52,6 +52,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Uses `X | None` union syntax for nullable columns
 - Supports all column types, foreign keys, indexes, and constraints
 
+**SQLAlchemy Relationships (issue #47)**
+- New `relationships` parameter for `create_models()` to generate `relationship()` with `back_populates`
+- Automatically generates bidirectional relationships for foreign keys:
+  - Parent side (one-to-many): collection attribute pointing to children
+  - Child side (many-to-one): attribute pointing to parent
+- Works with both `sqlalchemy` and `sqlalchemy_v2` model types
+- For `sqlalchemy_v2`: uses `Mapped[List[T]]` for one-to-many and `Mapped[T]` for many-to-one
+
 **SQLModel Improvements**
 - Fixed array type generation (issue #66)
 - Arrays now properly generate `List[T]` with correct SQLAlchemy ARRAY type

omymodels/from_ddl.py

@@ -44,6 +44,7 @@ def create_models(
     no_auto_snake_case: Optional[bool] = False,
     table_prefix: Optional[str] = "",
     table_suffix: Optional[str] = "",
+    relationships: Optional[bool] = False,
 ):
     """models_type can be: "gino", "dataclass", "pydantic" """
     # extract data from ddl file
@@ -65,6 +66,7 @@ def create_models(
         defaults_off,
         table_prefix=table_prefix,
         table_suffix=table_suffix,
+        relationships=relationships,
     )
     if dump:
         save_models_to_file(output, dump_path)
@@ -138,6 +140,56 @@ def save_models_to_file(models: str, dump_path: str) -> None:
         f.write(models)
 
 
+def _add_relationship(
+    relationships: Dict, table_name: str, fk_column: str, ref_table: str, ref_column: str
+):
+    """Helper to add both sides of a relationship."""
+    relationships.setdefault(table_name, []).append({
+        "type": "many_to_one",
+        "fk_column": fk_column,
+        "ref_table": ref_table,
+        "ref_column": ref_column,
+        "child_table_name": table_name,
+    })
+    relationships.setdefault(ref_table, []).append({
+        "type": "one_to_many",
+        "child_table": table_name,
+        "fk_column": fk_column,
+    })
+
+
+def _get_alter_columns(table) -> List:
+    """Get ALTER TABLE columns if they exist."""
+    if hasattr(table, 'alter') and table.alter:
+        return table.alter.get("columns", [])
+    return []
+
+
+def collect_relationships(tables: List) -> Dict:
+    """Collect foreign key relationships between tables."""
+    relationships = {}
+
+    for table in tables:
+        for column in table.columns:
+            if column.references and column.references.get("table"):
+                _add_relationship(
+                    relationships, table.name, column.name,
+                    column.references["table"],
+                    column.references.get("column") or column.name
+                )
+
+        for alter_col in _get_alter_columns(table):
+            ref_info = alter_col.get("references")
+            if ref_info and ref_info.get("table"):
+                _add_relationship(
+                    relationships, table.name, alter_col["name"],
+                    ref_info["table"],
+                    ref_info.get("column") or alter_col["name"]
+                )
+
+    return relationships
+
+
 def generate_models_file(
     data: Dict[str, List],
     singular: bool = False,
@@ -147,6 +199,7 @@ def generate_models_file(
     defaults_off: Optional[bool] = False,
     table_prefix: Optional[str] = "",
     table_suffix: Optional[str] = "",
+    relationships: Optional[bool] = False,
 ) -> str:
     """method to prepare full file with all Models &"""
     models_str = ""
@@ -159,6 +212,11 @@ def generate_models_file(
     if data["tables"]:
         add_custom_types_to_generator(data["types"], generator)
 
+        # Collect relationships if enabled
+        relationships_map = {}
+        if relationships:
+            relationships_map = collect_relationships(data["tables"])
+
         for table in data["tables"]:
             models_str += generator.generate_model(
                 table,
@@ -168,6 +226,7 @@ def generate_models_file(
                 defaults_off=defaults_off,
                 table_prefix=table_prefix,
                 table_suffix=table_suffix,
+                relationships=relationships_map.get(table.name, []) if relationships else [],
             )
         header += generator.create_header(
             data["tables"], schema=schema_global, models_str=models_str

omymodels/models/sqlalchemy/core.py

@@ -18,6 +18,7 @@ def __init__(self):
         self.postgresql_dialect_cols = set()
         self.constraint = False
         self.im_index = False
+        self.relationship_import = False
         self.types_mapping = types_mapping
         self.templates = st
         self.prefix = "sa."
@@ -46,14 +47,16 @@ def generate_model(
         singular: bool = True,
         exceptions: Optional[List] = None,
         schema_global: Optional[bool] = True,
+        relationships: Optional[List] = None,
         *args,
         **kwargs,
     ) -> str:
         """method to prepare one Model defention - name & tablename  & columns"""
         model = ""
+        model_name = create_class_name(table.name, singular, exceptions)
 
         model = st.model_template.format(
-            model_name=create_class_name(table.name, singular, exceptions),
+            model_name=model_name,
             table_name=table.name,
         )
         for column in table.columns:
@@ -62,8 +65,61 @@ def generate_model(
             )
         if table.indexes or table.alter or table.checks or not schema_global:
             model = logic.add_table_args(self, model, table, schema_global)
+
+        # Generate relationships if enabled
+        if relationships:
+            model += self._generate_relationships(
+                relationships, singular, exceptions
+            )
         return model
 
+    def _generate_relationships(
+        self,
+        relationships: List[Dict],
+        singular: bool,
+        exceptions: Optional[List] = None,
+    ) -> str:
+        """Generate relationship() lines for the model."""
+        result = "\n"
+        self.relationship_import = True
+
+        for rel in relationships:
+            if rel["type"] == "many_to_one":
+                # Child side: reference to parent
+                # e.g., posts.user = relationship("Users", back_populates="posts")
+                ref_table = rel["ref_table"]
+                fk_column = rel["fk_column"]
+                child_table_name = rel["child_table_name"]
+                related_class = create_class_name(ref_table, singular, exceptions)
+                # Attribute name derived from FK column (user_id -> user)
+                attr_name = fk_column.replace("_id", "") if fk_column.endswith("_id") else ref_table.lower()
+                # back_populates points to the collection on the parent (uses child table name)
+                back_pop_name = child_table_name.lower().replace("-", "_")
+                back_populates = st.back_populates_template.format(attr_name=back_pop_name)
+                result += st.relationship_template.format(
+                    attr_name=attr_name,
+                    related_class=related_class,
+                    back_populates=back_populates,
+                )
+            elif rel["type"] == "one_to_many":
+                # Parent side: collection of children
+                # e.g., users.posts = relationship("Posts", back_populates="user")
+                child_table = rel["child_table"]
+                fk_column = rel["fk_column"]
+                related_class = create_class_name(child_table, singular, exceptions)
+                # Attribute name is the child table name (as-is, since table names are typically plural)
+                attr_name = child_table.lower().replace("-", "_")
+                # back_populates points to the single parent reference on the child
+                # Derived from FK column (user_id -> user)
+                back_pop_name = fk_column.replace("_id", "") if fk_column.endswith("_id") else child_table.lower()
+                back_populates = st.back_populates_template.format(attr_name=back_pop_name)
+                result += st.relationship_template.format(
+                    attr_name=attr_name,
+                    related_class=related_class,
+                    back_populates=back_populates,
+                )
+        return result
+
     def create_header(
         self, tables: List[Dict], schema: bool = False, *args, **kwargs
     ) -> str:
@@ -82,4 +138,6 @@ def create_header(
             header += st.unique_cons_import + "\n"
         if self.im_index:
             header += st.index_import + "\n"
+        if self.relationship_import:
+            header += st.relationship_import + "\n"
         return header

omymodels/models/sqlalchemy/templates.py

@@ -51,3 +51,8 @@ class {model_name}(Base):\n
 
 on_delete = ', ondelete="{mode}"'
 on_update = ', onupdate="{mode}"'
+
+# relationship templates
+relationship_import = "from sqlalchemy.orm import relationship"
+relationship_template = '    {attr_name} = relationship("{related_class}"{back_populates})\n'
+back_populates_template = ', back_populates="{attr_name}"'

omymodels/models/sqlalchemy_v2/core.py

@@ -24,6 +24,7 @@ def __init__(self):
         self.time_import = False
         self.uuid_import = False
         self.fk_import = False
+        self.relationship_import = False
         self.types_mapping = types_mapping
         self.templates = st
         self.prefix = ""
@@ -225,6 +226,7 @@ def generate_model(
         singular: bool = True,
         exceptions: Optional[List] = None,
         schema_global: Optional[bool] = True,
+        relationships: Optional[List] = None,
         *args,
         **kwargs,
     ) -> str:
@@ -242,8 +244,68 @@ def generate_model(
         if table.indexes or table.alter or table.checks or not schema_global:
             model = self._add_table_args(model, table, schema_global)
 
+        # Generate relationships if enabled
+        if relationships:
+            model += self._generate_relationships(
+                relationships, singular, exceptions
+            )
+
         return model
 
+    def _generate_relationships(
+        self,
+        relationships: List[Dict],
+        singular: bool,
+        exceptions: Optional[List] = None,
+    ) -> str:
+        """Generate relationship() lines for the model."""
+        result = "\n"
+        self.relationship_import = True
+        self.typing_imports.add("List")
+
+        for rel in relationships:
+            if rel["type"] == "many_to_one":
+                # Child side: reference to parent
+                # e.g., author: Mapped["Authors"] = relationship("Authors", back_populates="books")
+                ref_table = rel["ref_table"]
+                fk_column = rel["fk_column"]
+                child_table_name = rel["child_table_name"]
+                related_class = create_class_name(ref_table, singular, exceptions)
+                # Attribute name derived from FK column (author_id -> author)
+                attr_name = fk_column.replace("_id", "") if fk_column.endswith("_id") else ref_table.lower()
+                # back_populates points to the collection on the parent (uses child table name)
+                back_pop_name = child_table_name.lower().replace("-", "_")
+                back_populates = st.back_populates_template.format(attr_name=back_pop_name)
+                # Type hint for many-to-one is the related class (quoted for forward ref)
+                type_hint = f'"{related_class}"'
+                result += st.relationship_template.format(
+                    attr_name=attr_name,
+                    type_hint=type_hint,
+                    related_class=related_class,
+                    back_populates=back_populates,
+                )
+            elif rel["type"] == "one_to_many":
+                # Parent side: collection of children
+                # e.g., books: Mapped[List["Books"]] = relationship("Books", back_populates="author")
+                child_table = rel["child_table"]
+                fk_column = rel["fk_column"]
+                related_class = create_class_name(child_table, singular, exceptions)
+                # Attribute name is the child table name (as-is, since table names are typically plural)
+                attr_name = child_table.lower().replace("-", "_")
+                # back_populates points to the single parent reference on the child
+                # Derived from FK column (author_id -> author)
+                back_pop_name = fk_column.replace("_id", "") if fk_column.endswith("_id") else child_table.lower()
+                back_populates = st.back_populates_template.format(attr_name=back_pop_name)
+                # Type hint for one-to-many is List of related class (quoted for forward ref)
+                type_hint = f'List["{related_class}"]'
+                result += st.relationship_template.format(
+                    attr_name=attr_name,
+                    type_hint=type_hint,
+                    related_class=related_class,
+                    back_populates=back_populates,
+                )
+        return result
+
     def _add_table_args(
         self, model: str, table: Dict, schema_global: bool = True
     ) -> str:
@@ -320,4 +382,7 @@ def create_header(
         if self.im_index:
             parts.append(st.index_import + "\n")
 
+        if self.relationship_import:
+            parts.append(st.relationship_import + "\n")
+
         return "".join(parts)

omymodels/models/sqlalchemy_v2/templates.py

@@ -60,3 +60,8 @@ class {model_name}(Base):
 
 on_delete = ', ondelete="{mode}"'
 on_update = ', onupdate="{mode}"'
+
+# relationship templates
+relationship_import = "from sqlalchemy.orm import relationship"
+relationship_template = '    {attr_name}: Mapped[{type_hint}] = relationship("{related_class}"{back_populates})\n'
+back_populates_template = ', back_populates="{attr_name}"'