Pirior to .Net 5：

你不能在Include或ThenInclude中使用where条件。你可以做的是：

var templatesFields = await _context.Sections
    .Include(x => x.Subtitles)
    .ThenInclude(r => r.Fields)
    .ThenInclude(r => r.OptionSources)
    .ThenInclude(r => r.OptionsSourcesDetails)
    .Where(t=>t.Subtitles.Fields.Any(x => x.TemplatesFields.TemplateID==TemplateID))
    .ToListAsync();

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使用.Net 5：

从.Net Core 5.0开始，这是可能的

var blogs = context.Blogs
    .Include(e => e.Posts.Where(p => p.Title.Contains("Cheese")))
    .ToList();

型
Filtered Include

从this：
不支持Include或ThenInclude中的过滤。使用Select创建投影：

questionnaire = _context.Questionnaires
    .Select(n => new Questionnaire
    {
        Id = n.Id,
        Name = n.Name,
        Questions = n.Questions.Select(q => new Question
        {
           Id = q.Id,
           Text = q.Text,
           Answers = q.Where(a => a.UserId == userId).ToList()
        }).ToList()
    })
    .FirstOrDefault(qn => qn.Id == questionnaireId);

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