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+# Study Session Data Model
+
+Study Copilot currently keeps sessions and progress in memory while the app is running. This document proposes a data model for generated study decks, card progress, bookmarks, and quiz attempts so future work can add persistence without changing the learning experience.
+
+## Current Behavior
+
+The FastAPI backend stores generated sessions in `study_sessions` and aggregate progress in `user_progress` in `backend/main.py`.
+
+Current session sources include:
+
+- `/api/generate-cards` for topic-based card generation.
+- `/api/upload-notes` for pasted notes.
+- `/api/upload-pdf` for extracted PDF text.
+
+The frontend keeps the active deck in `studyCardsList`, tracks the active `currentSessionId`, and calls backend endpoints for card views, bookmarks, quiz answers, progress, session history, and session reloads.
+
+## Goals
+
+- Preserve generated decks so learners can resume previous study sessions.
+- Track card-level progress without relying only on aggregate counters.
+- Support bookmarks, quiz attempts, and future progress analytics.
+- Avoid storing raw notes or extracted PDF text by default.
+- Leave room for future user accounts, wallet identity, and portable credentials.
+
+## Core Entities
+
+### StudySession
+
+Represents one generated deck from a topic, notes upload, or PDF upload.
+
+| Field | Type | Notes |
+| --- | --- | --- |
+| `id` | string | Stable session identifier. Avoid timestamp-only IDs in persistent storage. |
+| `user_id` | string or null | Optional until accounts exist. Can later map to wallet identity or auth user. |
+| `source_type` | enum | `topic`, `notes`, `pdf`, or `sandbox`. |
+| `source_title` | string | User-facing topic, notes title, or PDF filename. |
+| `difficulty` | enum | `beginner`, `intermediate`, or `advanced` when available. |
+| `card_count` | integer | Number of generated cards. |
+| `ai_provider` | string | Provider/model family used for the generation when known. |
+| `offline_mode` | boolean | Whether fallback cards were used. |
+| `created_at` | timestamp | Session creation time. |
+| `updated_at` | timestamp | Last progress or metadata update. |
+| `completed_at` | timestamp or null | Set when the learner finishes the deck. |
+| `deleted_at` | timestamp or null | Soft delete marker for privacy-friendly removal. |
+
+### StudyCard
+
+Represents a generated learning card within a session.
+
+| Field | Type | Notes |
+| --- | --- | --- |
+| `id` | string | Stable card identifier. |
+| `session_id` | string | Parent `StudySession.id`. |
+| `position` | integer | Zero-based or one-based order within the deck. |
+| `type` | enum | `concept`, `quiz`, `takeaway`, `challenge`, or `code`. |
+| `title` | string | Short learner-facing title. |
+| `content` | text | Main card body. |
+| `code_example` | text or null | Optional code block for code/challenge cards. |
+| `quiz_options` | JSON or related rows | Options for quiz cards. |
+| `created_at` | timestamp | Card generation time. |
+
+For relational storage, `quiz_options` can be normalized into a `QuizOption` table. For a small MVP, a JSON column is acceptable as long as validation enforces one correct answer.
+
+### CardProgress
+
+Represents a learner's interaction with one card.
+
+| Field | Type | Notes |
+| --- | --- | --- |
+| `id` | string | Stable progress row identifier. |
+| `user_id` | string or null | Optional until accounts exist. |
+| `session_id` | string | Parent session. |
+| `card_id` | string | Parent card. |
+| `first_viewed_at` | timestamp or null | First time the card became active. |
+| `last_viewed_at` | timestamp or null | Most recent view time. |
+| `view_count` | integer | Count of meaningful views, not every scroll event. |
+| `completed` | boolean | True when a learner reaches the card or answers it if quiz-based. |
+| `completed_at` | timestamp or null | Completion time. |
+
+### Bookmark
+
+Represents a learner-saved card.
+
+| Field | Type | Notes |
+| --- | --- | --- |
+| `id` | string | Stable bookmark identifier. |
+| `user_id` | string or null | Optional until accounts exist. |
+| `session_id` | string | Session containing the card. |
+| `card_id` | string | Saved card. |
+| `created_at` | timestamp | Bookmark time. |
+| `deleted_at` | timestamp or null | Allows undo/history without hard delete. |
+
+Use a unique constraint on `user_id + card_id` when accounts exist. Before accounts, use a browser session ID or temporary learner ID if persistence is introduced.
+
+### QuizAttempt
+
+Represents one submitted answer for a quiz card.
+
+| Field | Type | Notes |
+| --- | --- | --- |
+| `id` | string | Stable attempt identifier. |
+| `user_id` | string or null | Optional until accounts exist. |
+| `session_id` | string | Parent session. |
+| `card_id` | string | Quiz card. |
+| `selected_option` | string | Submitted option identifier or text. |
+| `is_correct` | boolean | Whether the answer matched the correct option. |
+| `answered_at` | timestamp | Submission time. |
+
+## Suggested Relationships
+
+```text
+StudySession 1 -> many StudyCard
+StudySession 1 -> many CardProgress
+StudyCard 1 -> many CardProgress
+StudyCard 1 -> many Bookmark
+StudyCard 1 -> many QuizAttempt
+User 1 -> many StudySession (future)
+User 1 -> many Bookmark (future)
+User 1 -> many QuizAttempt (future)
+```
+
+## API Shape
+
+The existing API can keep its current response shape while adding stable fields:
+
+- `POST /api/generate-cards` returns `session_id`, `cards`, `total_cards`, and `offline_mode`.
+- `POST /api/upload-notes` returns the same session shape.
+- `POST /api/upload-pdf` returns the same session shape plus `source_file`.
+- `POST /api/view/{card_id}` should eventually write or update `CardProgress`.
+- `POST /api/bookmark/{card_id}` should eventually toggle a `Bookmark` row.
+- `POST /api/quiz-answer` should eventually write a `QuizAttempt` row.
+- `GET /api/sessions` should list session summaries without returning full card bodies.
+- `GET /api/session/{session_id}` can return the full deck for resume.
+
+## Privacy And Retention
+
+- Do not store raw pasted notes or extracted PDF text by default.
+- Store generated cards and source metadata only when the learner expects session history.
+- Provide a delete path that removes sessions, cards, bookmarks, progress, and quiz attempts.
+- Avoid storing full provider prompts or raw AI responses unless explicitly needed for debugging.
+- If debugging logs are added, redact user material and expire logs quickly.
+- Treat PDF filenames and notes titles as potentially sensitive.
+- When wallet identity is added, avoid tying sensitive study content directly to public on-chain identifiers.
+
+## MVP Storage Recommendation
+
+For a first persistent version, use SQLite or Postgres with the entities above and keep raw source material out of the database. Store only:
+
+- session metadata,
+- generated cards,
+- card progress,
+- bookmarks,
+- quiz attempts.
+
+This keeps the product useful while reducing the privacy risk of retaining full study materials.
+
+## Migration Plan
+
+1. Replace timestamp-only session IDs with UUIDs.
+2. Add a storage adapter interface for sessions, cards, progress, bookmarks, and quiz attempts.
+3. Implement an in-memory adapter that preserves current behavior.
+4. Add a SQLite or Postgres adapter behind the same interface.
+5. Update endpoints to write through the adapter.
+6. Add tests for creating, listing, loading, bookmarking, viewing, answering, and deleting sessions.
+7. Add a user-facing setting for clearing local or persisted history.
+
+## Open Questions
+
+- Should anonymous learners get a temporary local user ID before account support exists?
+- How long should generated decks persist by default?
+- Should bookmarks survive if the parent session is deleted?
+- Should quiz attempts keep every answer or only the latest answer per card?
+- Should future wallet-linked credentials store only achievement proofs instead of study content?