What does it mean for a One-to-many Link relation to be Unique?

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i drug a line between two tables in the Linq Object Relational (O/R) mapper:

 Order                  Customer
--------------           ---------------
| OrderID    |           | CustomerID  |
| CustomerID |♦---------˃| ...         |
|            |           |             |
--------------           ---------------

Note: Or perhaps it was

 Order                    Customer
--------------           ---------------
| OrderID    |           | CustomerID  |
| CustomerID |˂---------♦| ...         |
| ...        |           |             |
--------------           ---------------

i’m not sure; it lets me drag both ways.

• First question, what’s the arrowhead, and what’s the diamond?

Assuming the second diagram, the cardinality of the Association was created as OneToMany. This makes sense, since:

• one customer
• has many orders

But what confuses me is the Association.Unique (Boolean) property. It defaults to false. This makes sense because it’s a OneToMany association. Order.CustomerID cannot be unique, otherwise it wouldn’t be a OneToMany association, it would be OneToOne.

But then i’m allowed to change the OneToMany Unique property to true. This makes no sense, so i conclude that Unique ness doesn’t apply to Order.CustomerID, but instead to Customer.CustomerID. But the diagram already indicates Customer.CustomerID is a Primary Key. Of course it’s unique, it’s a primary key.

But the Unique property isn’t set. This makes no sense, so i conclude that Unique ness doesn’t mean either table.

• Second question, what does Unique mean?

  Specifies whether the foreign target columns have a uniqueness constraint

• Third question: What is parent and child?

Assuming, again, the second diagram:

Customers.CustomerID ♦------------> Orders.CustomerID

i take Customers table to be a parent. It’s the one who owns what it means to be a customer. You want to change something about a customer, you walk to the parent.

Meanwhile, the child Orders table comes along and wants to reference a Customer.

     Parent(diamond)                     Child(arrowhead)
====================                     =================
Customers.CustomerID (PK) ♦------------> Orders.CustomerID (FK)

Except that when i look at the association’s Parent and Child properties:

Child property

• Name: Orders

Parent property

• Name: Customer

They want to create a property on the “child” called Orders. No, no, no. The child is orders. And they want to add a property to the parent called Customer. No, no no. The parent is a customer.

That means i must have it backwards, and the terms parent and child are the exact opposite of what i thought:

      Child(diamond)                     Parent(arrowhead)
====================                     =================
Customers.CustomerID (PK) ♦------------> Orders.CustomerID (FK)

And by this time i want to blow my brains out; and instead spend 35 minutes authoring a question on Stackoverflow; rather than continuing to scream at my computer.

Help.

See Answers

source: http://stackoverflow.com/questions/11057352/what-does-it-mean-for-a-one-to-many-link-relation-to-be-unique
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