Database Design with regard to my business logic - mysql

Question

i am building an invoicing application consisting of following business logic.
a) Place a new order for customer. (an order is a group of three
related components, estimate, invoice and purchaseorder)
b) After placing an order a new estimate can be generated. an order
will have only one estimate.(An estimate consist of item details)
c) With reference to an estimate of the order. an invoice can be
generated. an invoice qualify for the discount of price. apart from
item details, an invoice consist of some expenses.an order can contain
only one invoice
d) with reference to an invoice of the order, PurchaseOrder can be
generated. PurchaseOrder consist of item information about vendor
purchase. an order can contain multiple PurchaseOrder.
here is the database table design i have come up with.
while all look good, i am having difficulty deciding where to store the items list that belongs to particular estimate, invoice or purchaseorder of the order.
i had thought of several solution.
Approach A : create different tables for item list for each. (estimate, invoice and purchase order)
table : estimate_item , invoice_item , purchaseorder_item.(this tables contains columns similar to that of
order_item in above image).
problem: problem with this approach is all the three tables consist of identical columns storing identical information, the only
difference is foreign key that will be stored.
Approach B: create one item list table order_item
tablename: order_item
problem: not sure what to store as foreign key in this table since the foreign key can be from three different table. i thought of few
way of handling foreign keys in this table as follows.
1)foreignKey table reference column: type (example values: estimate, invoice, purchaseorder) foreignKey column: type_id(consist
of foreignKey of any of three tables) problem: i am using naming
convention for column names for example column name ending with tablename_id defines the foreign key. and this method violate the rules.
2) foreignKeyColumn: order_id , estimate_id , invoice_id ,
purchaseorder_id. problem: unnecessary foreign key columns
defined.
i want to know how should i store the foreign key in order_item table so that it identifies the order and estimate/invoice/purchaseorder it belongs too.
the relationship for tables are:
id is the primary key for all the tables
table name: order relates to (contact, estimate, invoice, shipment) tables.
column name: contact_id (foreign key(referring to id column of the contact table)).
column name: estimate_id (foreign key(referring to id column of the estimate table)).
column name: invoice_id (foreign key(referring to id column of the invoice table)).
column name: shipment_id (foreign key(referring to id column of the shipment table)).
tablename: purchaseorder (this have one to many relationship with order table)
column name: order_id (foreign key(referring to id column of the order table)).
column name: contact_id (foreign key(referring to id column of the contact table)).
the question is about how to go with the storing of foreign keys in order_item table.
Thank you.
Update 1:
Please note that each table estimate , invoice and purchaseorder will have item of it's own and having no relation to each other.

Answer 1

Hi I'm not sure how the relations happen. for instance, you have 'estimate' pointing to 'order item' but I don't see what key you have to make that join (or look-up). as another 'order' points to 'estimate' but how are those two joined? I dont see any shared attributes that both those entities have.
I'm assuming 'id' is just something to make rows in each particular table unique, but are not id's that have value to the application. so, I would think you' need to carry estimate.reference number into the 'order item' table. This is just a cursory comment.
also, would hep for clarity if keys were listed first. so in 'order item' you have attribute 'order id' (which appears to be an FK) buried in the end of the list of other attributes. makes this hard to read.

Answer 2

If I understand you correctly, each document associated with an order (i.e. an estimate, purchaseorder and/or invoice) might contain a different list of items.
If that is the case, I would probably create a Documents table along the following lines:
CREATE TABLE Documents (
DocumentId INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
OrderId INT NOT NULL,
-- you can move any fields common to all document types here
-- e.g. date created, reference #, etc.
FOREIGN KEY (OrderId) REFERENCES order (id)
);
And then your order_item, estimate, purchaseorder and invoice tables all reference their associated entry in this table:
ALTER TABLE [tablename]
ADD COLUMN DocumentId INT NOT NULL,
ADD FOREIGN KEY (DocumentId) REFERENCES Documents (DocumentId)
);
Is this what you're after?