Sheet
    course          CourseId
    name            (CI Text)
    description     Html Maybe
    type            SheetType
    grouping        SheetGroup
    markingText     Html Maybe
    visibleFrom     UTCTime Maybe
    activeFrom      UTCTime
    activeTo        UTCTime
    hintFrom        UTCTime Maybe
    solutionFrom    UTCTime Maybe
    uploadMode      UploadMode
    submissionMode  SheetSubmissionMode default='UserSubmissions'
    autoDistribute  Bool default=false
    CourseSheet course name
    deriving Generic
SheetEdit
    user            UserId
    time            UTCTime
    sheet           SheetId
SheetPseudonym
    sheet           SheetId
    pseudonym       Pseudonym
    user            UserId
    UniqueSheetPseudonym sheet pseudonym
    UniqueSheetPseudonymUser sheet user
SheetCorrector
    user            UserId
    sheet           SheetId
    load            Load
    state           CorrectorState default='CorrectorNormal'
    UniqueSheetCorrector user sheet
    deriving Show Eq Ord
SheetFile
    sheet           SheetId
    file            FileId
    type            SheetFileType
    UniqueSheetFile file sheet type