Support SQLAlchemy polymorphic / table inheritance

src/sqlalchemy_erd/constants/__init__.py

@@ -4,6 +4,7 @@
 - ``styling``: SVG card visual styling
 - ``routing``: orthogonal edge routing
 - ``layered``: the layered layout algorithm
+- ``relationships``: edge kinds and inheritance strategy labels
 
 Import from the relevant submodule, e.g. ``from sqlalchemy_erd.constants.geometry import NODE_W``.
 """

src/sqlalchemy_erd/constants/relationships.py

@@ -0,0 +1,7 @@
+"""Relationship edge kinds and inheritance strategy labels."""
+
+KIND_FK = "fk"
+KIND_INHERITANCE = "inheritance"
+
+STRATEGY_JOINED = "joined"
+STRATEGY_CONCRETE = "concrete"

src/sqlalchemy_erd/html_renderer.py

@@ -260,6 +260,10 @@ def render_html(
   el('polygon', {{ points:'0 0, 10 3.5, 0 7', fill: THEME.edgeColor }}, mk);
   const mkHi = el('marker', {{ id:'arr-hi', markerWidth:'10', markerHeight:'7', refX:'9', refY:'3.5', orient:'auto' }}, defs);
   el('polygon', {{ points:'0 0, 10 3.5, 0 7', fill: THEME.hiColor }}, mkHi);
+  const mkInherit = el('marker', {{ id:'inherit', markerWidth:'12', markerHeight:'10', refX:'11', refY:'5', orient:'auto' }}, defs);
+  el('polygon', {{ points:'1 1, 11 5, 1 9', fill: THEME.bgColor, stroke: THEME.edgeColor, 'stroke-width':'1.4' }}, mkInherit);
+  const mkInheritHi = el('marker', {{ id:'inherit-hi', markerWidth:'12', markerHeight:'10', refX:'11', refY:'5', orient:'auto' }}, defs);
+  el('polygon', {{ points:'1 1, 11 5, 1 9', fill: THEME.bgColor, stroke: THEME.hiColor, 'stroke-width':'1.8' }}, mkInheritHi);
 
   el('rect', {{ width:'100%', height:'100%', fill: THEME.bgColor }}, svg);
   el('rect', {{ width:'100%', height:'100%', fill:'url(#erd-dots)' }}, svg);
@@ -294,18 +298,20 @@ def render_html(
     }}
 
     const d = makePath(fp, fs, tp, ts);
+    const isInheritance = rel.kind === 'inheritance';
     const isNN = rel.fromCard === 'N' && rel.toCard === 'N';
     const isCross = Object.keys(SCHEMA_COLORS).length > 0 &&
       (fe.schema != null ? fe.schema : '_default') !== (te.schema != null ? te.schema : '_default');
 
-    const g = el('g', {{ 'data-rel-from': rel.from, 'data-rel-to': rel.to }}, svg);
+    const g = el('g', {{ 'data-rel-from': rel.from, 'data-rel-to': rel.to, 'data-rel-kind': rel.kind }}, svg);
     el('path', {{ d, fill:'none', stroke:'transparent', 'stroke-width':'18' }}, g);
     const edgeAttrs = {{
       class: 'erd-edge', d, fill:'none',
       stroke: THEME.edgeColor, 'stroke-width': '1.5',
-      'marker-end': 'url(#arr)'
+      'marker-end': isInheritance ? 'url(#inherit)' : 'url(#arr)'
     }};
-    if (isNN) edgeAttrs['stroke-dasharray'] = '5 3';
+    if (isInheritance) edgeAttrs['stroke-dasharray'] = '3 4';
+    else if (isNN) edgeAttrs['stroke-dasharray'] = '5 3';
     else if (isCross) edgeAttrs['stroke-dasharray'] = '8 4';
     el('path', edgeAttrs, g);
 
@@ -321,6 +327,13 @@ def render_html(
       fill: THEME.edgeColor, 'text-anchor':'middle', 'dominant-baseline':'middle',
       textContent: rel.toCard
     }}, g);
+    if (rel.label) {{
+      el('text', {{
+        class: 'erd-label', x: (fp[0] + tp[0]) / 2, y: (fp[1] + tp[1]) / 2 - 8,
+        'font-size':'10', 'font-family':'monospace', fill: THEME.edgeColor,
+        'text-anchor':'middle', 'dominant-baseline':'middle', textContent: rel.label
+      }}, g);
+    }}
   }}
 
   // Nodes — tagged with data attributes for updateHighlights
@@ -397,11 +410,13 @@ def render_html(
     const from = g.getAttribute('data-rel-from');
     const to = g.getAttribute('data-rel-to');
     const hi = hoveredId && (from === hoveredId || to === hoveredId);
+    const kind = g.getAttribute('data-rel-kind');
     const edge = g.querySelector('.erd-edge');
     if (edge) {{
       edge.setAttribute('stroke', hi ? THEME.hiColor : THEME.edgeColor);
       edge.setAttribute('stroke-width', hi ? '2' : '1.5');
-      edge.setAttribute('marker-end', `url(#${{hi ? 'arr-hi' : 'arr'}})`);
+      const marker = kind === 'inheritance' ? (hi ? 'inherit-hi' : 'inherit') : (hi ? 'arr-hi' : 'arr');
+      edge.setAttribute('marker-end', `url(#${{marker}})`);
     }}
     g.querySelectorAll('.erd-label').forEach(lbl => {{
       lbl.setAttribute('fill', hi ? THEME.hiColor : THEME.edgeColor);

src/sqlalchemy_erd/introspect.py

@@ -7,9 +7,13 @@
     Interval, JSON, LargeBinary, MetaData, Numeric, SmallInteger, String,
     Text, Time, UniqueConstraint, Uuid,
 )
-from sqlalchemy.orm import DeclarativeBase
+from sqlalchemy.orm import DeclarativeBase, Mapper
 from sqlalchemy.types import TypeDecorator
 
+from sqlalchemy_erd.constants.relationships import (
+    KIND_FK, KIND_INHERITANCE, STRATEGY_CONCRETE, STRATEGY_JOINED,
+)
+
 
 @dataclass
 class ColumnInfo:
@@ -36,6 +40,8 @@ class RelationshipInfo:
     from_card: str
     to_card: str
     fk_column: str
+    kind: str = KIND_FK
+    label: str | None = None
 
 
 @dataclass(frozen=True)
@@ -113,17 +119,18 @@ def _column_kind(col_info: Any, is_pk: bool, is_fk: bool) -> str:
 
 def _resolve_metadata(
     base_or_metadata: type[DeclarativeBase] | MetaData,
-) -> tuple[MetaData, dict[str, str]]:
-    """Return the metadata and a table-fullname → mapped-class-name lookup."""
+) -> tuple[MetaData, dict[str, str], list[Mapper]]:
+    """Return metadata plus mapper information when a DeclarativeBase is given."""
     if isinstance(base_or_metadata, MetaData):
-        return base_or_metadata, {}
+        return base_or_metadata, {}, []
 
     metadata = base_or_metadata.metadata
+    mappers = list(base_or_metadata.registry.mappers)
     class_names = {
         mapper.local_table.fullname: mapper.class_.__name__
-        for mapper in base_or_metadata.registry.mappers
+        for mapper in mappers
     }
-    return metadata, class_names
+    return metadata, class_names, mappers
 
 
 def _build_table(
@@ -208,6 +215,55 @@ def _build_relationships(
     return relationships
 
 
+def _inheritance_strategy(mapper: Mapper) -> str:
+    """Name the strategy of a child mapper that owns its own table."""
+    return STRATEGY_CONCRETE if mapper.concrete else STRATEGY_JOINED
+
+
+def _build_inheritance_relationships(
+    mappers: list[Mapper],
+    kept_names: set[str],
+) -> list[RelationshipInfo]:
+    relationships: list[RelationshipInfo] = []
+    for mapper in sorted(mappers, key=lambda m: m.local_table.fullname):
+        if mapper.inherits is None:
+            continue
+        parent_table = mapper.inherits.local_table
+        child_table = mapper.local_table
+        parent_name = parent_table.fullname
+        child_name = child_table.fullname
+        if parent_name == child_name:
+            continue
+        if parent_name not in kept_names or child_name not in kept_names:
+            continue
+
+        # The join column is the child PK that also references the parent.
+        fk_col = ""
+        pk_names = {col.name for col in child_table.primary_key.columns}
+        for col in child_table.columns:
+            references_parent = any(
+                fk.column.table.fullname == parent_name for fk in col.foreign_keys
+            )
+            if references_parent and col.name in pk_names:
+                fk_col = col.name
+                break
+        if not fk_col:
+            pk_cols = list(child_table.primary_key.columns)
+            fk_col = pk_cols[0].name if pk_cols else ""
+
+        strategy = _inheritance_strategy(mapper)
+        relationships.append(RelationshipInfo(
+            from_table=parent_name,
+            to_table=child_name,
+            from_card="1",
+            to_card="1",
+            fk_column=fk_col,
+            kind=KIND_INHERITANCE,
+            label=strategy,
+        ))
+    return relationships
+
+
 def _collapse_association_tables(
     filtered_items: list[tuple[str, Any]],
     tables: list[TableInfo],
@@ -256,7 +312,7 @@ def introspect_models(
     schemas: list[str] | None = None,
     filters: Filters | None = None,
 ) -> tuple[list[TableInfo], list[RelationshipInfo]]:
-    metadata, class_names = _resolve_metadata(base_or_metadata)
+    metadata, class_names, mappers = _resolve_metadata(base_or_metadata)
     filters = filters or Filters()
     include = _compile(filters.include_tables)
     exclude = _compile(filters.exclude_tables)
@@ -280,6 +336,17 @@ def introspect_models(
         filtered_items, tables, relationships,
     )
     kept_names = {t.name for t in tables}
+    inheritance_relationships = _build_inheritance_relationships(mappers, kept_names)
+    # Drop only the FK edge the inheritance edge replaces.
+    inheritance_edges = {
+        (rel.from_table, rel.to_table, rel.fk_column)
+        for rel in inheritance_relationships
+    }
+    relationships = [
+        rel for rel in relationships
+        if (rel.from_table, rel.to_table, rel.fk_column) not in inheritance_edges
+    ]
+    relationships.extend(inheritance_relationships)
     relationships = [
         rel for rel in relationships
         if rel.from_table in kept_names and rel.to_table in kept_names

src/sqlalchemy_erd/renderer.py

@@ -1,6 +1,7 @@
 from xml.sax.saxutils import escape
 
 from sqlalchemy_erd.constants.geometry import FIELD_H, HEADER_H, NODE_W, PAD
+from sqlalchemy_erd.constants.relationships import KIND_INHERITANCE
 from sqlalchemy_erd.constants.styling import (
     CARD_RADIUS, FIELD_FONT_SIZE, FIELD_INSET, HEADER_STRIP_H, KIND_FONT_SIZE,
     KIND_INSET, TITLE_FONT_SIZE, TITLE_INSET,
@@ -126,6 +127,9 @@ def render_svg(
     <marker id="arr-hi" markerWidth="10" markerHeight="7" refX="9" refY="3.5" orient="auto">
       <polygon points="0 0, 10 3.5, 0 7" fill="{theme.highlight_color}" />
     </marker>
+    <marker id="inherit" markerWidth="12" markerHeight="10" refX="11" refY="5" orient="auto">
+      <polygon points="1 1, 11 5, 1 9" fill="{theme.bg_color}" stroke="{theme.edge_color}" stroke-width="1.4" />
+    </marker>
   </defs>""")
 
     parts.append(f'  <rect width="100%" height="100%" fill="{theme.bg_color}" />')
@@ -164,9 +168,13 @@ def render_svg(
             tpt = _conn_pt(tp, tt, ts, to_idx, node_w)
 
         path_d = orthogonal_path(fpt, fs, tpt, ts)
+        is_inheritance = rel.kind == KIND_INHERITANCE
         is_nn = rel.from_card == "N" and rel.to_card == "N"
         is_cross = multi_schema and ft.schema != tt.schema
-        if is_nn:
+        marker = "inherit" if is_inheritance else "arr"
+        if is_inheritance:
+            dash = ' stroke-dasharray="3 4"'
+        elif is_nn:
             dash = ' stroke-dasharray="5 3"'
         elif is_cross:
             dash = ' stroke-dasharray="8 4"'
@@ -176,11 +184,14 @@ def render_svg(
         fl = _label_pos(fpt, fs)
         tl = _label_pos(tpt, ts)
 
-        parts.append(f'  <g class="erd-rel" data-from="{rel.from_table}" data-to="{rel.to_table}">')
+        parts.append(
+            f'  <g class="erd-rel" data-from="{rel.from_table}" data-to="{rel.to_table}" '
+            f'data-kind="{rel.kind}">'
+        )
         parts.append(f'    <path d="{path_d}" fill="none" stroke="transparent" stroke-width="18" />')
         parts.append(
             f'    <path class="erd-edge" d="{path_d}" fill="none" '
-            f'stroke="{theme.edge_color}" stroke-width="1.5"{dash} marker-end="url(#arr)" />'
+            f'stroke="{theme.edge_color}" stroke-width="1.5"{dash} marker-end="url(#{marker})" />'
         )
         parts.append(
             f'    <text x="{fl[0]}" y="{fl[1]}" font-size="11" '
@@ -192,6 +203,14 @@ def render_svg(
             f'font-family="monospace" fill="{theme.edge_color}" '
             f'text-anchor="middle" dominant-baseline="middle">{rel.to_card}</text>'
         )
+        if rel.label:
+            mx = (fpt[0] + tpt[0]) / 2
+            my = (fpt[1] + tpt[1]) / 2 - 8
+            parts.append(
+                f'    <text x="{mx}" y="{my}" font-size="10" '
+                f'font-family="monospace" fill="{theme.edge_color}" '
+                f'text-anchor="middle" dominant-baseline="middle">{escape(rel.label)}</text>'
+            )
         parts.append("  </g>")
 
     for table in tables:

src/sqlalchemy_erd/serialization.py

@@ -61,6 +61,8 @@ def build_relations_json(
             "fromCard": r.from_card,
             "toCard": r.to_card,
             "fkCol": r.fk_column,
+            "kind": r.kind,
+            "label": r.label,
         }
         for r in relationships
         if r.from_table in table_names and r.to_table in table_names

tests/conftest.py

@@ -1,4 +1,7 @@
-"""Shared fixtures for the ERDAlchemy test suite."""
+"""Shared fixtures for the ERDAlchemy test suite.
+
+Per-domain schemas live in ``tests/fixtures`` and are loaded as plugins.
+"""
 
 import pytest
 from sqlalchemy import (
@@ -10,6 +13,12 @@
 from datetime import datetime, timedelta, time
 from decimal import Decimal
 
+pytest_plugins = [
+    "tests.fixtures.inheritance",
+    "tests.fixtures.cardinality",
+    "tests.fixtures.comments",
+]
+
 
 # ── Single-table schema ──────────────────────────────────────────────────────
 
@@ -273,53 +282,3 @@ class Task(MultiFkBase):
 @pytest.fixture
 def multi_fk_base():
     return MultiFkBase
-
-
-# -- Cardinality schema -------------------------------------------------------
-
-cardinality_metadata = MetaData()
-
-Table(
-    "users", cardinality_metadata,
-    Column("id", Integer, primary_key=True),
-    Column("name", String(100), nullable=False),
-)
-
-Table(
-    "profiles", cardinality_metadata,
-    Column("user_id", Integer, ForeignKey("users.id"), primary_key=True),
-    Column("bio", Text),
-)
-
-Table(
-    "avatars", cardinality_metadata,
-    Column("id", Integer, primary_key=True),
-    Column("user_id", Integer, ForeignKey("users.id"), unique=True, nullable=False),
-)
-
-Table(
-    "tasks", cardinality_metadata,
-    Column("id", Integer, primary_key=True),
-    Column("assignee_id", Integer, ForeignKey("users.id"), nullable=True),
-)
-
-
-@pytest.fixture
-def cardinality_metadata_fixture():
-    return cardinality_metadata
-
-
-# -- Column comments schema ---------------------------------------------------
-
-comments_metadata = MetaData()
-
-Table(
-    "accounts", comments_metadata,
-    Column("id", Integer, primary_key=True),
-    Column("email", String(200), comment="Primary login email"),
-)
-
-
-@pytest.fixture
-def comments_metadata_fixture():
-    return comments_metadata

tests/fixtures/cardinality.py

@@ -0,0 +1,38 @@
+"""Fixtures for relationship cardinality schemas."""
+
+import pytest
+from sqlalchemy import Column, ForeignKey, Integer, MetaData, String, Table, Text
+
+
+# -- Cardinality schema -------------------------------------------------------
+
+cardinality_metadata = MetaData()
+
+Table(
+    "users", cardinality_metadata,
+    Column("id", Integer, primary_key=True),
+    Column("name", String(100), nullable=False),
+)
+
+Table(
+    "profiles", cardinality_metadata,
+    Column("user_id", Integer, ForeignKey("users.id"), primary_key=True),
+    Column("bio", Text),
+)
+
+Table(
+    "avatars", cardinality_metadata,
+    Column("id", Integer, primary_key=True),
+    Column("user_id", Integer, ForeignKey("users.id"), unique=True, nullable=False),
+)
+
+Table(
+    "tasks", cardinality_metadata,
+    Column("id", Integer, primary_key=True),
+    Column("assignee_id", Integer, ForeignKey("users.id"), nullable=True),
+)
+
+
+@pytest.fixture
+def cardinality_metadata_fixture():
+    return cardinality_metadata