Line items, dates and invitations

Most fields put one value in one column. Three do not, and each of them exists because the alternative is a person doing the same thing five times.

A table of line items

An invoice has lines; an order has items. Add a line items field in the form designer — Add field lists the tables that point at this one, under … as lines — and the dialog draws a small table: one row per record, a control per column, Add line, and a bin.

What it writes is rows of the other table, in the same transaction as the parent:

• The foreign key is the parent’s business. The field never shows it and never sends it.
• Saving an edit is a difference, not a rewrite: a line you changed is updated, a line you removed is deleted, and a line you left alone is not touched — so it keeps its id, its defaults and its history.
• Undo takes the lines with the record.
• Every line goes through the same column rules a row written on its own would.

You need permission on the other table. A grant that lets you write invoices does not let you write their lines, and a save that would need one says so instead of silently skipping them.

Totals

A line-items field can compute. Give it a row total — usually quantity times price — and a block of totals underneath: a subtotal, any number of rates (a tax, a levy) and a total. They add up as you type, over values nothing has saved yet.

Two rules worth knowing, because they are the two places money goes wrong:

• A rate is taken off the subtotal, never off the running total. A tax on a tax is a different number and nobody asked for one.
• Rounding happens once, at the end. Rounding each line first and adding the results gives a different answer, and it is the wrong one.

Nothing computed is stored unless a column holds it: name a column in the field and mark it read-only, and each line’s own total is written there. The block under the table is never stored — it is a reading of the lines.

A calendar, and the times of a day

A date or timestamp column can be shown as a calendar instead of a text box: a month grid, and — when the field defines opening times — a grid of slots beside it. The day and the time are one value in one column, so a form can never hold half a booking.

Set it in the field’s settings: Times from / until / every. Leave the times empty for a day picker with nothing beside it.

What “already taken” means

Nothing is crossed out until you say what taken means. Already taken when offers two answers:

Choice	What it checks
Any row holds that time	Another record already holds the same instant
A column	…and agrees on that column — the same room, the same person, the same machine

With a rule, Adminium asks once per visible month which instants are held, and crosses those out. A day is fully booked only when every one of its slots is taken.

Three things it deliberately does not do:

• It never crosses out the record you are editing. Its own slot is its own, or there would be no way to save it without moving it.
• A month with more bookings than it can check crosses out nothing, and says so. Half an answer drawn as a whole one marks free what it never looked at.
• It counts one row per slot. There is no capacity above one.

One record per entry

A chips field can be marked one record each: instead of a list in a column, every chip becomes its own row, sharing the rest of the form. Three email addresses in an invitations dialog are three invitations.

They are written in one transaction under one Undo: three rows or none. A list half sent, with no way back, is worse than one refused.

A summary box

A form can carry a recap: a sentence about what has been filled in, and one computed number, in the accent-tinted box the comps draw. It writes nothing and stores nothing — it reads the screen, which is why it can total a form nobody has saved yet.