code: async function list(ctx, next) {
    let post = ctx.state.post || ctx.query.post;
    let topic = ctx.state.topic || ctx.query.topic;
    if (!post && !topic) _2.default.error(400);
    let filters = ctx.state.filters || ctx.query.filters || {};
    if (post) {
      filters.post = post;
    } else if (topic) {
      filters.topic = topic;
    }
  
    ctx.state.filters = filters;
    await next();
  
    // $Flow 访问body
    let results = ctx.body.results;
    if (results) {
      // $Flow results一定有
      ctx.body.results = results.map(p => {
        if (p.user) {
          p.user = p.user.pick('id', 'username', 'avatar');
        }
        return p;
      });
    }
  }
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