Support for Composite Keys

[marination]

A composite key, in the context of relational databases, is a combination of two or more columns in a table that can be used to uniquely identify each row in the table. Uniqueness is only guaranteed when the columns are combined; when taken individually the columns do not guarantee uniqueness. (Src)

• Frappe currently supports only single column unique key constraints
• The current possibility for composite key implementations are hacky:
  1. Programatically handle this. I think this breaks the uniformity of where crucial parts of the schema are defined and just spreads it around.

     	def on_doctype_update():
     		frappe.db.add_unique("Bin", ["item_code", "warehouse"], constraint_name="unique_item_warehouse")

  2. Add an extra column C in your doctype that combines column A + column B (composite key) and add a uniqueness constraint on column C. This adds redundancy.

One challenge is the UI for this:

• I could visualise moving the uniqueness configuration outside the Fields table altogether
• Perhaps another table that supports specifiying 1 or more columns that form the constraints
• The Fields table could then have some basic visual indicator of which rows are unique or part of a uniqueness constraint