Add draft ERD schema definition in DBML format

Draft ERD schema definition using DBML for database structure.

Author: ayorush1

Documentation/'ERD schema definition (draft)'.md

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+// Use DBML to define your database structure
+// Docs: https://dbml.dbdiagram.io/docs
+
+Table users {
+  user_id integer [primary key]
+  username varchar
+  email string 
+  password string 
+  created_at datetime
+  rating int 
+}
+
+Table organisations {
+  organisation_id integer [primary key]
+  owner integer [ref: - users.user_id] 
+  created_at timestamp
+}
+
+Table members { 
+  user_id integer [ref: - users.user_id] 
+  organisation_id integer [ref: - organisations.organisation_id]
+  created_at timestamp 
+} 
+
+Table Inventory {
+  item_id integer [primary key]
+  owner integer [ref: - organisations.organisation_id]
+  name varchar 
+  description text 
+  quantity integer [not null]
+  available boolean
+}
+
+Table Loaned { 
+  transaction_id integer [primary key]
+  lender integer [ref: - organisations.organisation_id]
+  borrower integer [ref: - users.user_id]
+  item integer [ref: - Inventory.item_id]
+  loan_start datetime 
+  loan_end datetime 
+
+}
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