MetricApiController.cs

private IEnumerable<Resource> GetSchoolGroupAccesedResources(int? schoolId, SchoolGroup schoolGroup, DateTime startingDate)
{
    var schoolGroupResources = schoolGroup
                                .SchoolGroupPrograms
                                .SelectMany(sgp => sgp.EducationalProgram.EducationalProgramResources)
                                .Select(epr => epr.Resource)
                                .Where(r => r.UpdatedAt > startingDate);
    IEnumerable<int> schoolUserIds = null;
    if (schooId != null)
        schoolUserIds = school.Teachers.Select(t => t.TeacherId).Concat(school.Managers.Select(m => m.ManagerId);
    else
        schoolUserIds = schoolGroup.Schools.SelectMany(s => s.Teachers).Select(t => t.TeacherId)
                            .Concat(schoolGroup.Schools.SelectMany(s => s.Managers).Select(m => m.ManagerId))
    return schoolGroupResources.Where(r => r.ResourceAccesses.Any(ra => schoolUserIds.Contains(ra.UserId)));
}