-- =============================================
-- Education 模块 — 答案保存幂等性 DDL (PostgreSQL)
-- Ticket #7: 答案命令幂等、乐观锁并发控制、答案恢复
-- Migration: 003
-- Prerequisites: 002-education-practice-session.sql (session + question snapshots)
-- =============================================

-- =============================================
-- Preconditions
-- =============================================
-- Operator is expected to verify:
--   SELECT COUNT(*) FROM information_schema.tables
--     WHERE table_catalog = current_database()
--       AND table_name = 'education_answer_idempotency';
-- Result MUST be 0 before executing this migration.
--
-- Verify prerequisite tables exist:
--   SELECT COUNT(*) FROM information_schema.tables
--     WHERE table_catalog = current_database()
--       AND table_name IN ('education_practice_session', 'education_practice_question');
-- Result MUST be 2.

-- =============================================
-- 答案命令幂等表
-- =============================================
-- Purpose: Provide durable idempotency for answer save commands.
--   Same (tenant, user, operation, idempotency_key) + same request_hash → replay original response.
--   Same key + different request_hash → conflict.
--   Concurrent same-key inserts are resolved by unique constraint race handling.
--
-- Indexes:
--   uk_answer_idempotency — per-tenant, per-actor, per-operation uniqueness for idempotency key.
--     INSERT during answer save. DuplicateKeyException catch for concurrent-create race resolution.
--   idx_tenant_session — covers lookup by session for audit/debug.
--
-- response_json: Stores the serialized answer response for replay after network timeout/retry.
-- request_hash: SHA-256 of canonical payload (sorted JSON fields) for content-based dedup.
CREATE TABLE education_answer_idempotency (
    id               BIGINT       GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
    tenant_id        BIGINT       NOT NULL,
    user_id          BIGINT       NOT NULL,
    operation        VARCHAR(32)  NOT NULL DEFAULT 'SUBMIT_ANSWER',
    idempotency_key  VARCHAR(64)  NOT NULL,
    request_hash     VARCHAR(64)  NOT NULL,
    session_id       BIGINT       NOT NULL,
    question_id      VARCHAR(64)  NOT NULL,
    selected_answer  TEXT         DEFAULT NULL,
    status           VARCHAR(20)  NOT NULL DEFAULT 'ACCEPTED',
    response_json    TEXT         NOT NULL,
    creator          VARCHAR(64)  DEFAULT '',
    create_time      TIMESTAMP    NOT NULL DEFAULT CURRENT_TIMESTAMP,
    updater          VARCHAR(64)  DEFAULT '',
    update_time      TIMESTAMP    NOT NULL DEFAULT CURRENT_TIMESTAMP,
    deleted          BOOLEAN      NOT NULL DEFAULT false
);

COMMENT ON TABLE education_answer_idempotency IS '教育-答案命令幂等记录';
COMMENT ON COLUMN education_answer_idempotency.id IS '主键';
COMMENT ON COLUMN education_answer_idempotency.tenant_id IS '租户编号';
COMMENT ON COLUMN education_answer_idempotency.user_id IS '答题用户编号';
COMMENT ON COLUMN education_answer_idempotency.operation IS '操作类型：SUBMIT_ANSWER';
COMMENT ON COLUMN education_answer_idempotency.idempotency_key IS '客户端幂等键（UUID）';
COMMENT ON COLUMN education_answer_idempotency.request_hash IS '请求载荷 SHA-256 哈希';
COMMENT ON COLUMN education_answer_idempotency.session_id IS '会话 ID';
COMMENT ON COLUMN education_answer_idempotency.question_id IS '题目 ID';
COMMENT ON COLUMN education_answer_idempotency.selected_answer IS '学生已选答案';
COMMENT ON COLUMN education_answer_idempotency.status IS '状态：ACCEPTED-已接受, CONFLICT-冲突';
COMMENT ON COLUMN education_answer_idempotency.response_json IS '首次成功响应 JSON（用于重试重放）';
COMMENT ON COLUMN education_answer_idempotency.creator IS '创建者';
COMMENT ON COLUMN education_answer_idempotency.create_time IS '创建时间';
COMMENT ON COLUMN education_answer_idempotency.updater IS '更新者';
COMMENT ON COLUMN education_answer_idempotency.update_time IS '更新时间';
COMMENT ON COLUMN education_answer_idempotency.deleted IS '是否删除';

CREATE UNIQUE INDEX uk_answer_idempotency ON education_answer_idempotency (tenant_id, user_id, operation, idempotency_key);
CREATE INDEX idx_tenant_session ON education_answer_idempotency (tenant_id, session_id);

-- =============================================
-- PracticeQuestionDO: add client_sequence column
-- =============================================
-- Purpose: Track the last accepted client command sequence per question.
--   Rejects stale clientSequence: only sequences strictly greater than the
--   stored value are accepted (monotonic forward progression).
--   NULL means no answer has been accepted yet.
ALTER TABLE education_practice_question
    ADD COLUMN client_sequence INT DEFAULT NULL;
CREATE INDEX idx_client_sequence ON education_practice_question (client_sequence);

-- =============================================
-- PracticeSessionDO: add last_client_sequence column
-- =============================================
-- Purpose: Session-wide monotonic counter for client commands.
--   Rejects stale clientSequence across questions (not just per-question).
--   CAS incrementVersion now updates this column alongside version.
--   NULL means no answer has been accepted yet for this session.
ALTER TABLE education_practice_session
    ADD COLUMN last_client_sequence INT DEFAULT NULL;

-- =============================================
-- Post-migration verification queries
-- =============================================
-- Verify new table exists:
--   \d education_answer_idempotency
-- Verify unique key is enforced:
--   SELECT * FROM pg_indexes WHERE tablename = 'education_answer_idempotency' AND indexname = 'uk_answer_idempotency';
-- Verify column added to question table:
--   SELECT column_name, data_type, column_default
--     FROM information_schema.columns
--    WHERE table_catalog = current_database()
--      AND table_name = 'education_practice_question'
--      AND column_name = 'client_sequence';
-- Verify no orphan data (should be 0 after fresh migration):
--   SELECT COUNT(*) FROM education_answer_idempotency;