SchemaColumn.relationship(String name, ManagedPropertyType t, { bool isNullable: true, bool isUnique: false, String relatedTableName, String relatedColumnName, ManagedRelationshipDeleteRule rule: ManagedRelationshipDeleteRule.nullify })

Source 

SchemaColumn.relationship(this.name, ManagedPropertyType t,
    {this.isNullable: true,
    this.isUnique: false,
    this.relatedTableName,
    this.relatedColumnName,
    ManagedRelationshipDeleteRule rule:
        ManagedRelationshipDeleteRule.nullify}) {
  isIndexed = true;
  _type = typeStringForType(t);
  _deleteRule = deleteRuleStringForDeleteRule(rule);
}