Constraint Operations

`pgm.addConstraint( tablename, constraint_name, expression )`

Add a named column constraint - postgres docs

Arguments:

tablename [Name] - name of the table to alter
constraint_name [string] - name for the constraint
expression [string or object] - constraint expression (raw sql) or definition:
- check [string or array] - sql for a check constraint(s)
- unique [Name or array of Names or array of arrays of Names] - names of unique columns
- primaryKey [Name or array of Names] - names of primary columns
- exclude [string] - sql for an exclude constraint
- deferrable [boolean] - flag for deferrable table constraint
- deferred [boolean] - flag for initially deferred deferrable table constraint
- foreignKeys [object or array of objects] - foreign keys specification
  - columns [Name or array of Names] - names of columns
  - references [Name] - names of foreign table and column names
  - referencesConstraintName [string] - name of the created constraint
  - onDelete [string] - action to perform on delete
  - onUpdate [string] - action to perform on update
  - match [string] - FULL or SIMPLE

Aliases: createConstraint Reverse Operation: dropConstraint

`pgm.dropConstraint( tablename, constraint_name, options )`

Drop a named column constraint - postgres docs

Arguments:

tablename [Name] - name of the table to alter
constraint_name [string] - name for the constraint
options [object] - options:
- ifExists [boolean] - drops constraint only if it exists
- cascade [boolean] - drops also dependent objects

`pgm.renameConstraint( tablename, old_constraint_name, new_constraint_name )`

Rename a constraint - postgres docs

Arguments:

tablename [Name] - name of the table to alter
old_constraint_name [string] - current constraint name
new_constraint_name [string] - new constraint name

Reverse Operation: same operation in opposite direction