Database design with Loan and LoanLines - sql

Question

I have not been able to come up with a viable solution for days.
I am developing a system to maintain items and lending out these items.
Loan contains IEnumerable<LoanLine> which each points at an Item:
So far so good.
The tricky part comes to light when each item can't be lent out in the same period. And that period is defined by LoanLine.PickedUp ?? Loan.DateFrom > LoanLine.Returned ?? Loan.DateTo. This means that if LoanLine.PickedUp is null, then Loan.DateFrom should be used to compare, and if LoanLine.Returned is null, then Loan.DateTo should be used.
An item can be picked up and returned outside the loans boundaries. So these scenarioes can occur:
It should also be possible to "go back", ie. set LoanLine.Returned to null, in which case Loan.DateTo is used to compare again. The same goes with LoanLine.PickedUp.
It should also be possible to update both Loan.DateFrom and Loan.DateTo, with the berforementioned constraints still in effect. That means that if an update to Loan results in one of the lines, with either DateTime set to null, is overlapping, then the constraint shall throw an error.
This is the create-script:
create table loan
(
id int primary key identity(1, 1),
datefrom date not null,
dateto date not null,
employee_id int references employee(id) not null,
recipient_id int references employee(id) null,
note nvarchar(max) not null,
constraint c_loan_chkdates check (datefrom <= dateto)
);
create table loanlineitem
(
id int primary key identity(1, 1),
loan_id int references loan(id) on delete cascade not null,
item_id int references item(id) not null,
pickedup datetime null,
returned datetime null,
constraint uq_loanlineitem unique (loan_id, item_id),
constraint c_loanlineitem_chkdates check (returned is null or pickedup <= returned)
);
And this is the constraint:
create function checkLoanLineItem(#itemId int, #loanId int, #pickedup datetime, #returned datetime)
returns bit
as
begin
declare #result bit = 0;
declare #from date = #pickedup;
declare #to date = #returned;
--If either #from or #to is null, fill the ones with null from loan-table
if (isnull(#from, #to) is null)
begin
select #from = isnull(#from, datefrom),
#to = isnull(#to, dateadd(d, 1, dateto))
from loan
where id = #loanId;
end
if not exists (select top 1 lli.id from loanlineitem lli
inner join loan l on lli.loan_id = l.id
where l.id <> #loanId
and lli.item_id = #itemId
and ((isnull(lli.pickedup, l.datefrom) >= #from and isnull(lli.pickedup, l.datefrom) < #to)
--When comparing datetime with date, the date's time is 00:00:00
--so one day is added to account for this
or (isnull(lli.returned, dateadd(d, 1, l.dateto)) >= #from and isnull(lli.returned, dateadd(d, 1, l.dateto)) < #to))
)
begin
set #result = 1;
end
return #result;
end;
go;
alter table loanlineitem
add constraint c_loanlineitem_checkoverlap check (dbo.checkLoanLineItem(item_id, loan_id, pickedup, returned) = 1)
go;
I could make a similar constraint on Loan-table but then I would have similar code two places, which I would prefer to avoid, if possible.
So what I'm asking is; should I rethink my schema to accomplish this, or is it possible with some constraints which I'm not familiar with?

Answer 1

For this we will need two things:
A way to track the status of an item with respect to a loan
Only allow one active loan at a point in time
The first item can be addressed through the data model (see below) but the second will require any changes to the database MUST occur through stored procedures and those stored procedures will have to contain logic to keep the database in a consistent state. Otherwise you'll have a real mess on your hands (or rely on triggers, which is another headache).
We'll track the physical state of the item through an item status based on a timestamp, and, if desired, reservations through another mechanism based on a future date.
This query will return the current status and loan of all items, as well as the next reservation. From this you can also determine which items are past due.
SELECT
Item.ItemId
,ItemStatus.UpdateDtm
,ItemStatus.StatusCd
,ItemStatus.LoanNumber
,Loan.StartDt
,Loan.EndDt
,Reservation.StartDt
,Reservation.EndDt
FROM
Item Item
LEFT JOIN
LoanItemStatus ItemStatus
ON ItemStatus.ItemId = Item.ItemId
AND ItemStatus.UpdateDtm =
(
SELECT
MAX(UpdateDtm)
FROM
LoanItemStatus
WHERE
ItemId = Item.ItemId
)
LEFT JOIN
Loan Loan
ON Loan.LoanNumber = ItemStatus.LoanNumber
LEFT JOIN
ItemReservation Reservation
ON Reservation.ItemId = Item.ItemId
AND Reservation.StartDt =
(
SELECT
MIN(StartDt)
FROM
ItemReservation
WHERE
ItemId = Item.ItemId
AND StartDt >= GetDate()
)
It will probably make sense to harden this logic into a view.
To see if an item is reserved during a given timeframe:
SELECT
Item.ItemId
,CASE
WHEN COALESCE(PriorReservation.EndDt,GETDATE()) <= #ReservationStartDt AND #ReservationEndDt <= COALESCE(NextReservation.StartDt,'9999-12-31') THEN 'Y'
ELSE 'N'
END AS ReservationAvailableInd
FROM
Item Item
LEFT JOIN
ItemReservation PriorReservation
ON PriorReservation.ItemId = Item.ItemId
AND PriorReservation.StartDt =
(
SELECT
MAX(StartDt)
FROM
ItemReservation
WHERE
ItemId = Item.ItemId
AND StartDt <= #ReservationStartDt
)
LEFT JOIN
ItemReservation NextReservation
ON NextReservation.ItemId = Item.ItemId
AND NextReservation.StartDt =
(
SELECT
MIN(StartDt)
FROM
ItemReservation
WHERE
ItemId = Item.ItemId
AND StartDt > #ReservationStartDt
)
So you'll need to roll all of this into your stored procedures so:
When an item is loaned, it is available for the time period specified
When the loan date range is changed it does not conflict with the existing items or future reservations
When new reservations are made they do not conflict with existing procedures reservations
State transitions make sense (Not loaned/Returned -> Awaiting pickup -> Picked Up -> Returned/Lost)
You cannot delete loans with items that have been picked up or items that have been picked up

Answer 2

Alright, I found the solution, although it isn't the most elegant or DRY one.
First a view with occupations (thanks bbaird for the suggestion, which made it easier to figure out the logic):
create view vw_loanlineitem_occupations
as
select lli.id, loan_id, item_id, isnull(lli.pickedup, l.datefrom) as [from], isnull(lli.returned, dateadd(d, 1, l.dateto)) as [to] from loanlineitem lli inner join loan l on lli.loan_id = l.id
then a general check overlap udf:
create function udf_isOverlapping(#span1Start datetime, #span1End datetime, #span2Start datetime, #span2End datetime)
returns bit
as
begin
return iif((#span1Start <= #span2End and #span1End >= #span2Start), 1, 0);
end
then a udf and constraint on loan:
create function udf_isLoanValid(#loanId int, #dateFrom date, #dateTo date)
returns bit
as
begin
declare #result bit = 0;
--When type 'date' is compared to 'datetime' the time-part is 00:00:00, so add one day
set #dateTo = dateadd(d, 1, #dateTo)
if not exists (
select top 1 lli.id from loanlineitem lli
inner join loan l on lli.loan_id = l.id
--Only check items that are in this loan
where lli.item_id in (select item_id from loanlineitem where loan_id = #loanId)
--Check if this span is overlapping with other lines/loans
--When type 'date' is compared to 'datetime' the time-part is 00:00:00, so add one day
and (dbo.udf_isOverlapping(
#dateFrom,
#dateTo,
isnull(lli.pickedup, iif(l.id = #loanId, #dateFrom, l.datefrom)),
isnull(lli.returned, iif(l.id = #loanId, #dateTo, dateadd(d, 1, l.dateto)))
) = 1
)
)
begin
set #result = 1
end
return #result;
end;
go;
alter table loan
add constraint c_loan_datecheck check (dbo.udf_isLoanValid(id, dateFrom, dateTo) = 1);
and a separate constraint on loanlineitem, which unfortunately repeats some of the code from loan's constraint:
create function udf_isLineValid(#itemId int, #loanId int, #pickedup datetime, #returned datetime)
returns bit
as
begin
declare #result bit = 0;
declare #from date = #pickedup;
declare #to date = #returned;
--If either #from or #to is null, fill the ones with null from loan-table
if (#from is null or #to is null)
begin
select #from = isnull(#from, datefrom),
#to = isnull(#to, dateadd(d, 1, dateto))
from loan
where id = #loanId;
end
--If no lines with overlap exists, this line is valid, so set result to 1
if not exists (
select top 1 id from vw_loanlineitem_occupations
where item_id = #itemId
and loan_id <> #loanId
and dbo.udf_isOverlapping(#from, #to, [from], [to]) = 1
)
begin
set #result = 1;
end
return #result;
end;
go;
alter table loanlineitem
add constraint c_loanlineitem_checkoverlap check (dbo.udf_isLineValid(item_id, loan_id, pickedup, returned) = 1)
It works, which is the most important part. I am not sure about how performance is, but data integrity is more important.