how to form DataRelations between DataTables when there's duplicate values in columns in C#/.NET 2.0

Question

i have a situation like this:

TOWN_PLAN_UNIT table:
ID  NAME
123 Block123
456 Block345

BUILDING_TOWN_PLAN_UNIT table:
UNIT_ID BUILDING_ID
123     999
123     908
456     333
456     999

BUILDING
ID  NAME
999 Building999

I want to gather all data of block456 to a tree and I use DataRelations:
TOWN_PLAN_UNIT.ID -> BUILDING_TOWN_PLAN_UNIT.UNIT_ID
BUILDING_TOWN_PLAN_UNIT.BUILDING_ID -> BUILDING.ID

i've been using true as last parameter to DataRelation constructor (create constrains) and I get exception when second relation is created because building with id 999 is for both blocks. Should I just use false? Can I still get childrows? Or is there some finer-grain way to control this?

Cheers & BR- Matti