SqlAlchemy Query.All() 意外返回 OperationalError

Question

我正在开发的“Reddit 克隆”Web 应用程序中使用 Elasticsearch 实现搜索功能。我想支持搜索线程、用户和 subreddits，但是当我输入搜索查询并搜索上述 3 个不包含任何匹配项的类别之一时，我得到一个意外的“OperationalError”而不是一个空的结果集。

如我包含的代码所示，我尝试使用返回以下错误的 sqlalchemy.orm.query.Query.all() 函数：

OperationalError: (sqlite3.OperationalError) near "END": syntax error
[SQL: SELECT user.id AS user_id, user.username AS user_username, user.email AS user_email, user.password_hash AS user_password_hash, user.last_sign_in AS user_last_sign_in 
FROM user 
WHERE 1 != 1 ORDER BY CASE user.id END]
(Background on this error at: http://sqlalche.me/e/e3q8)

我研究了其他 StackOverflow 帖子，发现 first() 函数在内部处理数据库结果，如果没有找到结果则返回 None，但是当我切换到该函数时，我遇到了这个错误：

OperationalError: (sqlite3.OperationalError) near "END": syntax error
[SQL: SELECT user.id AS user_id, user.username AS user_username, user.email AS user_email, user.password_hash AS user_password_hash, user.last_sign_in AS user_last_sign_in 
FROM user 
WHERE 1 != 1 ORDER BY CASE user.id END
 LIMIT ? OFFSET ?]
[parameters: (1, 0)]
(Background on this error at: http://sqlalche.me/e/e3q8)

查看 SqlAlchemy 的文档，我在这两个函数中都没有看到任何关于此错误的提及，并且阅读了 OperationalError 的含义，我担心我的数据库设置可能不正确。

app/routes.py：这是处理对以下 URL 的搜索请求的路由：http://localhost:5000/search?q=&index=

@app.route('/search', methods=['GET'])
def search():
    print 'Hit the /search route!'
    if not g.search_form.validate():
        return redirect(url_for('index'))
    page = request.args.get('page', 1, type=int)
    target_index = request.args.get('index', 'thread')
    if target_index == 'thread':
        results, total = Thread.search(g.search_form.q.data, page, app.config['POSTS_PER_PAGE'])
        print 'Called Thread.search(), total results = {}'.format(total['value'])
    elif target_index == 'user':
        results, total = User.search(g.search_form.q.data, page, app.config['POSTS_PER_PAGE'])
        print 'Called User.search(), total results = {}'.format(total['value'])
    elif target_index == 'subreddit':
        results, total = Subreddit.search(g.search_form.q.data, page, app.config['POSTS_PER_PAGE'])
        print 'Called Subreddit.search(), total results = {}'.format(total['value'])
    else:
        return render_template('404.html')
    try:
        results = results.all()
    except OperationalError:
        results = [None]
    total = total['value']
    next_url = url_for('search', index=target_index, q=g.search_form.q.data, page=page + 1) if total > page * app.config['POSTS_PER_PAGE'] else None
    prev_url = url_for('search', index=target_index, q=g.search_form.q.data, page=page - 1) if page > 1 else None
    results_list = zip(results, [None] * len(results)) # Temporarily to match expected input for template
    return render_template('search.html', title=_('Search'), results_list=results_list, next_url=next_url, prev_url=prev_url, query=g.search_form.q.data, index=target_index)

app/models.py：

class SearchableMixin(object):
    @classmethod
    def search(cls, expression, page, per_page):
        ids, total = query_index(cls.__tablename__, expression, page, per_page)
        if total == 0:
            return cls.query.filter_by(id=0), 0
        when = []
        for i in range(len(ids)):
            when.append((ids[i], i))
        return cls.query.filter(cls.id.in_(ids)).order_by(
            db.case(when, value=cls.id)), total

    @classmethod
    def before_commit(cls, session):
        session._changes = {
            'add': list(session.new),
            'update': list(session.dirty),
            'delete': list(session.deleted)
        }

    @classmethod
    def after_commit(cls, session):
        for obj in session._changes['add']:
            if isinstance(obj, SearchableMixin):
                add_to_index(obj.__tablename__, obj)
        for obj in session._changes['update']:
            if isinstance(obj, SearchableMixin):
                add_to_index(obj.__tablename__, obj)
        for obj in session._changes['delete']:
            if isinstance(obj, SearchableMixin):
                remove_from_index(obj.__tablename__, obj)
        session._changes = None

    @classmethod
    def reindex(cls):
        for obj in cls.query:
            add_to_index(cls.__tablename__, obj)

db.event.listen(db.session, 'before_commit', SearchableMixin.before_commit)
db.event.listen(db.session, 'after_commit', SearchableMixin.after_commit)

# Below is one model that implements SearchableMixin to allow searching # for users. Thread and Subreddit models follow the same logic.
class User(db.Model, UserMixin, SearchableMixin):
    __searchable__ = ['username']
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(64), index=True, unique=True)
    # <Remaining User model fields here...>

app/search.py​​：（保存底层搜索函数以查询 Elasticsearch 索引）

def add_to_index(index, model):
    if not app.elasticsearch:
        return
    payload = {}
    for field in model.__searchable__:
        payload[field] = getattr(model, field)
    app.elasticsearch.index(index=index, doc_type=index, id=model.id,
                                    body=payload)

def remove_from_index(index, model):
    if not app.elasticsearch:
        return
    app.elasticsearch.delete(index=index, doc_type=index, id=model.id)

def query_index(index, query, page, per_page):
    if not app.elasticsearch:
        return [], 0
    search = app.elasticsearch.search(
        index=index,
        body={'query': {'multi_match': {'query': query, 'fields': ['*']}},
              'from': (page - 1) * per_page, 'size': per_page})
    ids = [int(hit['_id']) for hit in search['hits']['hits']]
    return ids, search['hits']['total']

正如我所包含的 app/routes.py 所示，我通过捕获 OperationalError 并将其视为未找到结果的指示器来解决此问题，但由于 all() 文档没有提及它，所以我没想到会引发此异常。

Answer 1

我通过将您检索到的所有字段隐藏在星号后面来稍微简化了生成的查询。

SELECT user.*
FROM user 
WHERE 1 != 1
ORDER BY CASE user.id END

首先，只要1 != 1 是where 子句，此查询将不会返回任何值，因为根据定义，这是错误的。 ids 有可能是空的吗？这也可以很好地解释格式错误的CASE 语句，这是错误的根源。通常情况下，case(dict(a=1, b=2), value=User.name) 应该会导致 CASE WHEN name = 'a' THEN 1 WHEN name = 'b' THEN 2 END，它会正确执行。