How to detect collision on specific body?

Hi everyone.

Here’s my problem:

I have the following function to add physical bodys:

function addSensor(x, y, width, height)     x = x + width / 2     y = y +height / 2     Sensor[currentSensor] = display.newRect(x,y,width,height)     Sensor[currentSensor].x = x     Sensor[currentSensor].y = y     Sensor[currentSensor].width = width     Sensor[currentSensor].height = height     Sensor[currentSensor]:setFillColor(80,80,80)     Sensor[currentSensor].ID = "obstacle"     physics.addBody(Sensor[currentSensor], "static", { bounce = 0, friction = 1.0, filter = {maskBits =5, categoryBits = 2} } )     currentSensor = currentSensor + 1 end  

It’s called like this:

addSensor (200,0,50,50) addSensor (300,100,50,50) addSensor (400,200,50,50)  

My previous collision detection looked like this:

local function onCollision(event)     if (event.phase == "began") then         if (event.object1.ID == "obstacle") or (event.object2.ID == "obstacle") then             if (event.object1.ID == "balloon") or (event.object2.ID == "balloon") then                 balloon.paused = true                 balloon.canJump = true                 collX, collY, collW, collH = obstacle.x, obstacle.y, obstacle.width, obstacle.height                 limit = (collW + collH) / 4                 if limit \> 120 then limit = 120 end             end         end     end end  

The problem is, all the physical objects have the same “ID” (obstacle).

How do i find out on which of them the collision occured?

Best regards.