Constraints

Constraints are just queries ending in one of the constraint statements like :unique, :fk and :check. These operators throw exceptions when the constraints are not met.

You can use constraints to get confidence that databases are always in valid states, and you didn't accidentally screw something up (perhaps at dev time).

To constrain a database such that you get errors when putting the db into invalid states, you materialize constraint queries (and they can be removed with demat).

Constraints can apply to any query, so you can apply constraints to aggregates and joins, here is the obligatory order can have at most 10 items if its associated customer is called bob and its tuesday constraint.

[[:from Order] 
 [:join Customer {:customer-id :customer-id}]
 [:where [= "bob" :firstname] [tuesday? [rel/env :now]]]
 [:check {:pred [<= [count :items] 10],
          :error [str "order can have at most 10 items if its associated customer is called bob and its tuesday, found: " [count :items]]}]]

As it is convenient to specify multiple constraints on a query in one form, a special :constrain operator is provided.

e.g

[[:from Customer]
 [:constrain
   [:check [string? :firstname] [string? :lastname] [nat-int? :age]]
   [:unique :customer-id]
   [:fk Address {:address-id :address-id}]]]

Constraint operators

• :check ensure certain predicates hold
• :req ensure cols exist
• :fk ensure a referenced row exists in some other query/table
• :constrain combine multiple constraints on a query/table
• :unique unsure only one row exists for some set of expressions