diff --git a/backend/Dockerfile b/backend/Dockerfile
index d0080e5..2c9718c 100644
--- a/backend/Dockerfile
+++ b/backend/Dockerfile
@@ -15,4 +15,4 @@ RUN mkdir -p /app/data
 
 EXPOSE 8000
 
-CMD ["sh", "-c", "alembic upgrade head && uvicorn app.main:app --host 0.0.0.0 --port 8000"]
+CMD ["sh", "-c", "alembic upgrade head && python repair_class_memberships.py && uvicorn app.main:app --host 0.0.0.0 --port 8000"]
diff --git a/backend/alembic/versions/20260426_remove_legacy_roster_and_user_columns.py b/backend/alembic/versions/20260426_remove_legacy_roster_and_user_columns.py
index 2db9249..7dbfd9f 100644
--- a/backend/alembic/versions/20260426_remove_legacy_roster_and_user_columns.py
+++ b/backend/alembic/versions/20260426_remove_legacy_roster_and_user_columns.py
@@ -9,6 +9,8 @@ from __future__ import annotations
 
 from alembic import op
 import sqlalchemy as sa
+from app.db.base import Base
+from app.db import models as _models  # noqa: F401 ensure metadata registered
 
 
 revision = "20260426_remove_legacy"
@@ -17,15 +19,137 @@ branch_labels = None
 depends_on = None
 
 
+def _ensure_class_memberships_table(inspector: sa.Inspector) -> None:
+    tables = set(inspector.get_table_names())
+    if "class_memberships" in tables:
+        return
+
+    op.create_table(
+        "class_memberships",
+        sa.Column("id", sa.Integer(), autoincrement=True, nullable=False),
+        sa.Column("user_id", sa.Integer(), nullable=False),
+        sa.Column("class_id", sa.Integer(), nullable=False),
+        sa.Column("membership_role", sa.String(length=20), nullable=False, server_default="student"),
+        sa.Column("committee_role", sa.String(length=50), nullable=True),
+        sa.Column("class_permissions", sa.Text(), nullable=True),
+        sa.Column("created_at", sa.DateTime(), server_default=sa.func.now(), nullable=False),
+        sa.Column("updated_at", sa.DateTime(), server_default=sa.func.now(), nullable=False),
+        sa.ForeignKeyConstraint(["user_id"], ["users.id"]),
+        sa.ForeignKeyConstraint(["class_id"], ["classes.id"]),
+        sa.PrimaryKeyConstraint("id"),
+        sa.UniqueConstraint("user_id", "class_id", name="uq_class_membership_user_class"),
+    )
+    op.create_index("ix_class_memberships_user_id", "class_memberships", ["user_id"])
+    op.create_index("ix_class_memberships_class_id", "class_memberships", ["class_id"])
+
+
+def _existing_memberships(bind) -> set[tuple[int, int]]:
+    rows = bind.execute(
+        sa.text("SELECT user_id, class_id FROM class_memberships")
+    ).fetchall()
+    return {(int(user_id), int(class_id)) for user_id, class_id in rows}
+
+
+def _insert_membership(
+    bind,
+    existing: set[tuple[int, int]],
+    *,
+    user_id: int,
+    class_id: int,
+    membership_role: str,
+    committee_role: str | None,
+    class_permissions: str | None,
+    created_at,
+    updated_at,
+) -> None:
+    key = (user_id, class_id)
+    if key in existing:
+        return
+
+    bind.execute(
+        sa.text(
+            """
+            INSERT INTO class_memberships (
+                user_id, class_id, membership_role, committee_role, class_permissions, created_at, updated_at
+            ) VALUES (
+                :user_id, :class_id, :membership_role, :committee_role, :class_permissions, :created_at, :updated_at
+            )
+            """
+        ),
+        {
+            "user_id": user_id,
+            "class_id": class_id,
+            "membership_role": membership_role,
+            "committee_role": committee_role,
+            "class_permissions": class_permissions,
+            "created_at": created_at,
+            "updated_at": updated_at,
+        },
+    )
+    existing.add(key)
+
+
 def upgrade() -> None:
     bind = op.get_bind()
+    Base.metadata.create_all(bind=bind, checkfirst=True)
     inspector = sa.inspect(bind)
+    _ensure_class_memberships_table(inspector)
 
     tables = set(inspector.get_table_names())
+    existing = _existing_memberships(bind)
+
+    user_columns = {column["name"] for column in inspector.get_columns("users")}
+    if "class_id" in user_columns:
+        legacy_rows = bind.execute(
+            sa.text(
+                """
+                SELECT id, class_id, role, committee_role, class_permissions, created_at, updated_at
+                FROM users
+                WHERE class_id IS NOT NULL
+                """
+            )
+        ).mappings()
+        for row in legacy_rows:
+            if row["role"] == "super_admin":
+                continue
+            _insert_membership(
+                bind,
+                existing,
+                user_id=int(row["id"]),
+                class_id=int(row["class_id"]),
+                membership_role="teacher" if row["role"] == "teacher" else "student",
+                committee_role=row["committee_role"],
+                class_permissions=row["class_permissions"],
+                created_at=row["created_at"],
+                updated_at=row["updated_at"],
+            )
+
+    if "student_rosters" in tables:
+        roster_rows = bind.execute(
+            sa.text(
+                """
+                SELECT user_id, class_id, created_at
+                FROM student_rosters
+                WHERE user_id IS NOT NULL
+                """
+            )
+        ).mappings()
+        for row in roster_rows:
+            _insert_membership(
+                bind,
+                existing,
+                user_id=int(row["user_id"]),
+                class_id=int(row["class_id"]),
+                membership_role="student",
+                committee_role=None,
+                class_permissions=None,
+                created_at=row["created_at"],
+                updated_at=row["created_at"],
+            )
+
     if "student_rosters" in tables:
         op.drop_table("student_rosters")
 
-    user_columns = {column["name"] for column in inspector.get_columns("users")}
     legacy_columns = {"class_id", "committee_role", "class_permissions"}
     if legacy_columns.intersection(user_columns):
         with op.batch_alter_table("users") as batch_op:
@@ -38,6 +162,17 @@ def upgrade() -> None:
 
 
 def downgrade() -> None:
+    bind = op.get_bind()
+    inspector = sa.inspect(bind)
+    tables = set(inspector.get_table_names())
+    if "class_memberships" in tables:
+        indexes = {index["name"] for index in inspector.get_indexes("class_memberships")}
+        if "ix_class_memberships_class_id" in indexes:
+            op.drop_index("ix_class_memberships_class_id", table_name="class_memberships")
+        if "ix_class_memberships_user_id" in indexes:
+            op.drop_index("ix_class_memberships_user_id", table_name="class_memberships")
+        op.drop_table("class_memberships")
+
     with op.batch_alter_table("users") as batch_op:
         batch_op.add_column(sa.Column("class_id", sa.Integer(), nullable=True))
         batch_op.add_column(sa.Column("committee_role", sa.String(length=50), nullable=True))
diff --git a/backend/alembic/versions/20260427_create_class_memberships_if_missing.py b/backend/alembic/versions/20260427_create_class_memberships_if_missing.py
new file mode 100644
index 0000000..3ac98a2
--- /dev/null
+++ b/backend/alembic/versions/20260427_create_class_memberships_if_missing.py
@@ -0,0 +1,54 @@
+"""create class memberships table if missing
+
+Revision ID: 20260427_create_memberships
+Revises: 20260426_remove_legacy
+Create Date: 2026-04-27 10:45:00
+"""
+
+from __future__ import annotations
+
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = "20260427_create_memberships"
+down_revision = "20260426_remove_legacy"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    bind = op.get_bind()
+    inspector = sa.inspect(bind)
+
+    if "class_memberships" not in set(inspector.get_table_names()):
+        op.create_table(
+            "class_memberships",
+            sa.Column("id", sa.Integer(), autoincrement=True, nullable=False),
+            sa.Column("user_id", sa.Integer(), nullable=False),
+            sa.Column("class_id", sa.Integer(), nullable=False),
+            sa.Column("membership_role", sa.String(length=20), nullable=False, server_default="student"),
+            sa.Column("committee_role", sa.String(length=50), nullable=True),
+            sa.Column("class_permissions", sa.Text(), nullable=True),
+            sa.Column("created_at", sa.DateTime(), server_default=sa.func.now(), nullable=False),
+            sa.Column("updated_at", sa.DateTime(), server_default=sa.func.now(), nullable=False),
+            sa.ForeignKeyConstraint(["user_id"], ["users.id"]),
+            sa.ForeignKeyConstraint(["class_id"], ["classes.id"]),
+            sa.PrimaryKeyConstraint("id"),
+            sa.UniqueConstraint("user_id", "class_id", name="uq_class_membership_user_class"),
+        )
+        op.create_index("ix_class_memberships_user_id", "class_memberships", ["user_id"])
+        op.create_index("ix_class_memberships_class_id", "class_memberships", ["class_id"])
+
+
+def downgrade() -> None:
+    bind = op.get_bind()
+    inspector = sa.inspect(bind)
+    tables = set(inspector.get_table_names())
+    if "class_memberships" in tables:
+        indexes = {index["name"] for index in inspector.get_indexes("class_memberships")}
+        if "ix_class_memberships_class_id" in indexes:
+            op.drop_index("ix_class_memberships_class_id", table_name="class_memberships")
+        if "ix_class_memberships_user_id" in indexes:
+            op.drop_index("ix_class_memberships_user_id", table_name="class_memberships")
+        op.drop_table("class_memberships")
diff --git a/backend/check_membership_health.py b/backend/check_membership_health.py
new file mode 100644
index 0000000..d44af0e
--- /dev/null
+++ b/backend/check_membership_health.py
@@ -0,0 +1,56 @@
+"""
+Check class membership health after migration / repair.
+
+Usage:
+    python check_membership_health.py
+Or inside Docker:
+    docker compose exec backend python check_membership_health.py
+"""
+
+from __future__ import annotations
+
+import asyncio
+
+from sqlalchemy import func, select
+
+from app.db.database import async_session
+from app.db.models import ClassMembership, Class_, User
+
+
+async def main() -> None:
+    async with async_session() as db:
+        total_users = (await db.execute(select(func.count(User.id)))).scalar() or 0
+        total_classes = (await db.execute(select(func.count(Class_.id)))).scalar() or 0
+        total_memberships = (await db.execute(select(func.count(ClassMembership.id)))).scalar() or 0
+
+        users_without_membership = (
+            await db.execute(
+                select(User.id, User.name, User.email, User.role)
+                .outerjoin(ClassMembership, ClassMembership.user_id == User.id)
+                .where(User.role != "super_admin")
+                .group_by(User.id)
+                .having(func.count(ClassMembership.id) == 0)
+            )
+        ).all()
+
+        classes_without_members = (
+            await db.execute(
+                select(Class_.id, Class_.name)
+                .outerjoin(ClassMembership, ClassMembership.class_id == Class_.id)
+                .group_by(Class_.id)
+                .having(func.count(ClassMembership.id) == 0)
+            )
+        ).all()
+
+        print(f"users={total_users} classes={total_classes} memberships={total_memberships}")
+        print(f"users_without_membership={len(users_without_membership)}")
+        for row in users_without_membership[:20]:
+            print("  user_without_membership", row)
+
+        print(f"classes_without_members={len(classes_without_members)}")
+        for row in classes_without_members[:20]:
+            print("  class_without_members", row)
+
+
+if __name__ == "__main__":
+    asyncio.run(main())
diff --git a/backend/repair_class_memberships.py b/backend/repair_class_memberships.py
new file mode 100644
index 0000000..8e0322b
--- /dev/null
+++ b/backend/repair_class_memberships.py
@@ -0,0 +1,104 @@
+"""
+Repair missing class_memberships data for databases that were partially migrated.
+
+Usage:
+    python repair_class_memberships.py
+Or inside Docker:
+    docker compose exec backend python repair_class_memberships.py
+"""
+
+from __future__ import annotations
+
+import asyncio
+from collections import defaultdict
+
+from sqlalchemy import select, text
+
+from app.db.database import async_session, engine
+from app.db.base import Base
+from app.db.models import (
+    Announcement,
+    Assignment,
+    AssignmentSubmission,
+    ClassMembership,
+    FundRecord,
+    Resource,
+    Timeline,
+    User,
+    Vote,
+)
+
+
+async def ensure_class_memberships_table() -> None:
+    async with engine.begin() as conn:
+        await conn.run_sync(Base.metadata.create_all, tables=[ClassMembership.__table__])
+
+
+async def repair_memberships() -> None:
+    await ensure_class_memberships_table()
+
+    async with async_session() as db:
+        existing_result = await db.execute(
+            select(ClassMembership.user_id, ClassMembership.class_id)
+        )
+        existing = {(row[0], row[1]) for row in existing_result.all()}
+
+        inferred: dict[tuple[int, int], str] = {}
+
+        def remember(user_id: int | None, class_id: int | None, role: str) -> None:
+            if not user_id or not class_id:
+                return
+            key = (user_id, class_id)
+            current = inferred.get(key)
+            if current == "teacher":
+                return
+            inferred[key] = "teacher" if role == "teacher" else (current or "student")
+
+        user_rows = await db.execute(select(User.id, User.role))
+        user_roles = {user_id: role for user_id, role in user_rows.all()}
+
+        statement_specs = [
+            (Announcement.author_id, Announcement.class_id),
+            (Timeline.author_id, Timeline.class_id),
+            (Resource.uploader_id, Resource.class_id),
+            (Vote.creator_id, Vote.class_id),
+            (Assignment.creator_id, Assignment.class_id),
+            (FundRecord.recorder_id, FundRecord.class_id),
+        ]
+
+        for user_col, class_col in statement_specs:
+            rows = await db.execute(select(user_col, class_col))
+            for user_id, class_id in rows.all():
+                remember(user_id, class_id, user_roles.get(user_id, "student"))
+
+        submission_rows = await db.execute(
+            select(AssignmentSubmission.student_id, Assignment.class_id).join(
+                Assignment, Assignment.id == AssignmentSubmission.assignment_id
+            )
+        )
+        for user_id, class_id in submission_rows.all():
+            remember(user_id, class_id, user_roles.get(user_id, "student"))
+
+        grouped = defaultdict(list)
+        for (user_id, class_id), role in inferred.items():
+            if (user_id, class_id) not in existing:
+                grouped[role].append((user_id, class_id))
+
+        created = 0
+        for role, pairs in grouped.items():
+            for user_id, class_id in pairs:
+                db.add(
+                    ClassMembership(
+                        user_id=user_id,
+                        class_id=class_id,
+                        membership_role="teacher" if role == "teacher" else "student",
+                    )
+                )
+                created += 1
+
+        await db.commit()
+        print(f"repair_class_memberships: created {created} memberships")
+
+
+if __name__ == "__main__":
+    asyncio.run(repair_memberships())