Context and Problem Statement

Users want to add custom metadata to items. For example, for invoices fields like invoice-number or total/amount make sense. When using a pagination stamp, every item gets a pagination number.

This is currently not possible to realize in docspell. But it is an essential part when organizing and archiving documents. It should be supported.

Considered Options

Requirements

Fields have simple types: There is a difference in presenting a date, string or number. At least some simple types should be distinguishable for the UI to make it more convenient to use.
An item can have at most one value of a field: The typical example is invoice number – it doesn't make sense to be able to specify two invoice-numbers on an item. If still necessary, one can create artificial fields like invoice-number-1 and invoice-number-2.
Fulltext Index: should custom field values be sent to the full-text index?
- This is not required, imho. At least not for a start.
Fields are stored per collective. When creating a new field, user can select from existing ones to avoid creating same fields with different names.
Fields can be managed: Rename, change type, delete. Show fields that don't have any value associated and could be deleted.

Ideas

Database

Initial sketch:

CREATE TABLE custom_field (
  id varchar(244) not null primary key,
  name varchar(100) not null,
  cid varchar(254) not null,
  ftype varchar(100) not null,
  foreign key "cid" references collective(cid),
  unique (cid, name)
);

CREATE TABLE custom_field_item_value (
  id varchar(254) not null primary key,
  item_id varchar(254) not null,
  field varchar(254) not null,
  field_value varchar(254),
  foreign key item_id references item(id),
  foreign key field references custom_field(id),
  unique (item_id, field) -- only one value allowed per item
)

field carries the type in the column ftype. type is an enum: text, numeric, date, money, bool
the type is just some constant, the database doesn't care and can't enforce anything
the field name is unique per collective
a value to a field can only exist on an item
only one value per item can be created for one field
the values are represented as a string in the database
the application is responsible for converting into a string
date is a local date, the iso format is used (e.g. 2020-08-11)
Why not each type a separate column, like value_str, value_date etc?
- making them different requires to fetch all fields first before running a query, in order to know which columns to check
  - usually the query would look like this: my_field_1 == "test"; in order to know what column to check for my_field_1, a query to fetch the field must be done first. Only then the type is known and its clear what column to use for the value. This complicates searching and increases query count.
- The value must be present (or converted) into the target type
- It's a lot simpler for the implementation to reduce every custom field to a string value at the database. Type-specific queries (like avg, sum etc) can still be done using sql CAST function.

Changing Type:

change the type on the custom_field table
the string must be convertible to the new type, which must be ensured by the application

Adding more types:

ammend the ftype enum with a new value and provide conversion functions

REST Api

the known field types must be transferred to the ui
the ui creates custom presentation for date, money etc

Input 1:

setting one value for a specific field. The server knows its type and converts accordingly (e.g. string->date)
json only knows number, strings and bools (and null).

make a structure to allow to specify all json types:

{ value_str: Maybe String
, value_num: Maybe Float
, value_bool: Maybe Bool
}

con: setting all to Nothing is an error as well as using the wrong field with some type (e.g. setting value_str for setting a numeric field)
con: very confusing – what to use for fields of type "date" or "money"?
client needs some parsing anyways to show errors

Input 2:

send one value as a string
```
{ value: String
}
```
string must be in a specific format according to its type. server may convert (like 12.4999 → 12.49), or report an error
client must create the correct string

Output:

server sends field name, type and value per custom field. Return an array of objects per item.

Searching:

UI knows all fields of a collective: user selects one in a dropdown and specifies the value

Decision Outcome

values are strings at the database
values are strings when transported from/to server
client must provide the correct formatted strings per type
- numeric: some decimal number
- money: decimal number
- text: no restrictions
- date: a local date as iso string, e.g. 2011-10-09
- bool: either "true" or "false", case insensitive

Initial Version

create the database structure and a REST api to work with custom fields
create a UI on item detail to add/set custom fields
show custom fields on item detail
create a page to manage fields: only rename and deletion
extend the search for custom fields
show custom fields in search results

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