Dotnet Core Web API返回空列表|延迟加载不起作用

我的API中有一个端点,它返回选定项的所有数据。该项是一个名为 Survey 它有一个页面列表。

public partial class Surveys
{
    public Surveys()
    {
        Pages = new HashSet<Pages>();
    }

    public string Description { get; set; }
    public string Name { get; set; }
    public long Syear { get; set; }
    public long Quarter { get; set; }
    public ICollection<Pages> Pages { get; set; }
}

Pages 像这样。

public partial class Pages
{
    public Pages()
    {
        Elements = new HashSet<Elements>();
    }

    public string Id { get; set; }
    public long Number { get; set; }
    public string Name { get; set; }
    public string Description { get; set; }
    public int? PageFlowId { get; set; }
    public bool NamedPage { get; set; }
    public bool? IsFirst { get; set; }
    public bool? IsLast { get; set; }
    public long? SurveyQuarter { get; set; }
    public long? SurveySyear { get; set; }

    public PagesFlows PageFlow { get; set; }
    public Surveys Survey { get; set; }
    public ICollection<Elements> Elements { get; set; }
}

页

[
    {
        "description": "Customer Satisfaction Survey",
        "name": "Customer Survey",
        "syear": 2019,
        "quarter": 1,
        "pages": []
    }
]

数据库包含数据。表的主键是复合键(Syear,Quarter)。我的API看起来像这样。

public async Task<IActionResult> GetSurveys([FromRoute]long syear, long quarter)
    {
        if (!ModelState.IsValid)
        {
            return BadRequest(ModelState);
        }
        var surveys = await _context.Surveys.Include(s => s.Pages).SingleOrDefaultAsync(s => s.Syear == syear && s.Quarter == quarter);
        if (surveys == null)
        {
            return NotFound();
        }

        return Ok(surveys);
    }

我已经想了一个星期了。如有任何帮助,将不胜感激,提前谢谢。