I am using grails GORM for database mapping and oracle as database which is already populated with values.My question is can I define a new constraint like Foreign key in the domain class even if its not defined in the underlying table in my legacy db?The grails app is still going to accept the constraints right?The constraints dont have to exactly match those of database right?
Yes, the constraints you define in your domain class will be respected regardless of what you have in your database. The only time the 2 really relate is if you are letting Hibernate generate DDL for you (which most folks do not do for their production environment) in which case there are certain constraints which affect the DDL that is generated. Since you already have a database you almost certainly have that turned off.
EDIT:
An example of a constraint which affects DDL is the size constraint. If you constrain a String field with something like size: 5..15, by default the DDL that is generated will create a column that is 15 characters wide. If you are not allowing the app to generate DDL that constraint is still applied at validation time and if the property has more than 15 characters or fewer than 5, validation will fail. Once validation passes and the data is sent to the database, the framework assumes everything will be ok there. If it isn't, then corresponding exceptions may be thrown. For example, if the String has 12 characters it will pass validation in the app and will be sent to the database. If the database column is only 8 characters wide, you are going to get a SQLException. I hope that makes sense.