logic for placing object was not good

logic was supposed to allow placing object on farms, which it didn't

now it does.

+ changed ObjectPlaceholder's name so it follows the TilePlaceholder's convention

Assets/Scripts/Drag&Drop/GameObjectPlacementButton.cs

@@ -12,7 +12,7 @@ public class GameObjectPlacementButton : UnitPlacementButton
         foreach (var comp in instance.transform.GetAllComponents<Collider2D>()) Destroy(comp);
         foreach (var comp in instance.transform.GetAllComponents<MonoBehaviour>()) Destroy(comp);
 
-        var placeholder = instance.AddComponent<ObjectDraggablePlaceholder>();
+        var placeholder = instance.AddComponent<ObjectPlaceholder>();
         placeholder.Prefab = _prefab;
 
         var outline = new GameObject("Outline");

Assets/Scripts/Drag&Drop/ObjectDraggablePlaceholder.cs

@@ -1,10 +0,0 @@
-using UnityEngine;
-
-public class ObjectDraggablePlaceholder : DraggablePlaceholder
-{
-    public GameObject Prefab { get; set; }
-    public override void Place()
-    {
-        Prefab.Create(transform.position);
-    }
-}

Assets/Scripts/Drag&Drop/ObjectPlaceholder.cs

@@ -0,0 +1,18 @@
+using UnityEngine;
+
+public class ObjectPlaceholder : DraggablePlaceholder
+{
+    public GameObject Prefab { get; set; }
+    public override void Place()
+    {
+        Prefab.Create(transform.position);
+    }
+    public override bool CanBePlacedHere()
+    {
+        var objsOnPosition = LevelManager.Instance.GetAll<ILevelObject>(obj => obj.Position.IsContainedIn(transform.position));
+        var positionContainsFarm = objsOnPosition.Exists(obj => obj.Name.ToLower().Contains("farm"));
+
+        //either no object, or object is farm
+        return objsOnPosition.Count == 0 || (objsOnPosition.Count == 1 && positionContainsFarm);
+    }
+}