relation = None
for testedRelation in Document.Data.Relations:
    if(testedRelation.Expression == jsonParam['expression']):
        relation = testedRelation
        break
if(relation == None):
    Document.Data.Relations.Add(Document.Data.Tables[jsonParam['leftColumn']['tableName']], Document.Data.Tables[jsonParam['rightColumn']['tableName']], jsonParam['expression'])
