1. An ontology modeler implemented in a data processing system, the ontology modeler comprising: a class builder for generating and editing classes within an ontology model, wherein the class builder generates classes within the ontology model by executing computer usable program code: to identify, by a processor, a first table within a source relational database schema, the first table comprising at least one first column and at least one first row; to identify, by a processor, a second table within a target relational database schema, the second table comprising at least one second column and at least one second row; and to map, by a processor, the first table and the second table onto a common ontology model, wherein the first table is a first ontological class within the common ontology model, and wherein the second table is a second ontological class within the common ontology model; a property builder for generating and editing properties of classes within the ontology model, wherein the property builder generates properties of the classes within the ontology model by executing computer usable program code: to identify, by a processor, each of the at least one first column as a set properties of the first class, and to identify, by a processor, each of the at least one second column as a set properties of the second class; a business rules builder for generating and editing business rules involving properties of classes within the ontology model, wherein the business rules generates business rules within the ontology model by executing computer usable program code: to express, by a processor, dependencies among the set of properties of the first class, dependencies among the set of properties of the second class, and dependencies between the set of properties of the first class and the set of properties of the second class, wherein the business rules are selected from the group consisting of arithmetic dependencies among numeric data, string dependencies among character string data, and look-up tables; and a test instance builder for generating and editing test instances of classes in the ontology model, wherein said test instance builder includes a test instance builder user interface for indicating values for properties of a test instance, and a test instance validator for validating test instances, wherein said test instance validator checks for inconsistencies with regard to property values of test instances.