observers should use filter_with

addons/ob.luau

@@ -22,11 +22,12 @@ local function observers_new<T...>(
 ): Observer<T...>
 	query:cached()
 
+
 	local world = (query :: Query<T...> & { world: World }).world
 	callback = callback
 
 	local archetypes = {}
-	local terms = query.ids
+	local terms = query.filter_with :: { jecs.Id }
 	local first = terms[1]
 
 	local observers_on_create = world.observable[jecs.ArchetypeCreate][first]
@@ -42,6 +43,10 @@ local function observers_new<T...>(
 
 	local entity_index = world.entity_index :: any
 
+	for _, archetype in query:archetypes() do
+		archetypes[archetype.id] = true
+	end
+
 	local function emplaced<T, a>(
 		entity: jecs.Entity<T>,
 		id: jecs.Id<a>,
@@ -132,6 +137,10 @@ local function monitors_new<T...>(query: Query<T...>): Observer<T...>
 	observer_on_delete.callback = function(archetype)
 		archetypes[archetype.id] = nil
 	end
+
+	for _, archetype in query:archetypes() do
+		archetypes[archetype.id] = true
+	end
 	local entity_index = world.entity_index :: any
 
 	local callback_added: ((jecs.Entity) -> ())?

test/addons/ob.luau

@@ -7,6 +7,23 @@ local ob = require("@addons/ob")
 
 TEST("addons/ob", function()
 	local world = jecs.world()
+	do CASE "should match against archetypes that were already created"
+		local A = world:component()
+
+		local e1 = world:entity()
+		world:add(e1, A)
+
+		local monitor = ob.monitor(world:query(A))
+		local c = 1
+		monitor.added(function()
+			c += 1
+		end)
+
+		world:remove(e1, A)
+		world:add(e1, A)
+		CHECK(c==2)
+	end
+
 	do CASE "Should support query:without()"
 		local A = world:component()
 		local B = world:component()

test/tests.luau

@@ -1365,7 +1365,6 @@ TEST("world:added", function()
 	end
 
 	do CASE ""
-		local world = jecs.world()
 		local IsNearby = world:component()
 		world:set(IsNearby, jecs.Name, "IsNearby")
 		local person1, person2 = world:entity(), world:entity()