Expand the How To series

Author: Ukendio

how_to/004_tags.luau

@@ -43,6 +43,12 @@ world:add(entity, Dead) -- Adds the tag
 
 print(world:has(entity, Dead)) -- true
 
+-- jecs.is_tag(world, id) returns true if the id is a tag (no data).
+print(jecs.is_tag(world, Dead)) -- true
+
+local Position = world:component() :: jecs.Id<number>
+print(jecs.is_tag(world, Position)) -- false
+
 -- Tags are removed using world:remove(entity, component)
 
 world:remove(entity, Dead)

how_to/012_entity_liveliness.luau

@@ -2,7 +2,7 @@
 	Relationships makes it possible to describe entity graphs natively in ECS.
 
 	Adding/removing relationships is similar to adding/removing regular components,
-	with as difference that instead of a single component id, a relationship adds
+	with the difference that instead of a single component id, a relationship adds
 	a pair of two things to an entity. In this pair, the first element represents
 	the relationship (e.g. "Eats"), and the second element represents the relationship
 	target (e.g. "Apples").
@@ -36,9 +36,6 @@ world:add(alice, pair(Likes, bob))
 -- Test if entity has a relationship pair
 print(world:has(bob, pair(Eats, Apples)))  -- true
 
--- Test if entity has a relationship wildcard
-print(world:has(bob, pair(Eats, jecs.Wildcard)))  -- true
-
 --[[
 	Querying for relationship targets
 
@@ -77,26 +74,6 @@ for child in world:query(pair(ChildOf, parent)) do
 	print(`Entity {child} is a child of parent {parent}`)
 end
 
---[[
-	Querying with wildcards and getting targets
-
-	When you query with a wildcard, you can use world:target() to get the
-	actual target entity. This is useful when you want to find all entities
-	with a relationship, regardless of the target.
-]]
-
--- Find all entities that eat something (any target)
-for entity in world:query(pair(Eats, jecs.Wildcard)) do
-	local food = world:target(entity, Eats)  -- Get the actual target
-	print(`Entity {entity} eats {food}`)
-end
-
--- Find all entities that like someone (any target)
-for entity in world:query(pair(Likes, jecs.Wildcard)) do
-	local target = world:target(entity, Likes)
-	print(`Entity {entity} likes {target}`)
-end
-
 --[[
 	Combining relationship queries with regular components
 
@@ -118,39 +95,6 @@ for entity, pos, health in world:query(Position, Health, pair(ChildOf, parent))
 	print(`Child {entity} has position {pos} and health {health}`)
 end
 
---[[
-	Querying for entities with multiple relationship targets
-
-	An entity can have multiple relationships with the same relationship type
-	but different targets. For example, bob might like both alice and charlie.
-
-	When querying with a wildcard, you'll get the entity once, but world:target()
-	will return the first matching target. If you need all targets, you'll need
-	to use a different approach (see the targets example for advanced usage).
-]]
-
-local charlie = world:entity()
-world:add(bob, pair(Likes, charlie))
-
--- This query will return bob once, even though bob likes both alice and charlie
-for entity in world:query(pair(Likes, jecs.Wildcard)) do
-	local target = world:target(entity, Likes)
-	print(`Entity {entity} likes {target}`)  -- Will show one target per entity
-end
-
---[[
-	Querying for all relationships with a specific target
-
-	You can also query for all entities that have any relationship with a
-	specific target using a wildcard for the relationship part.
-]]
-
--- Find all entities that have any relationship with alice as the target
-for entity in world:query(pair(jecs.Wildcard, alice)) do
-	-- Note: This is less common and may have performance implications
-	print(`Entity {entity} has some relationship with alice`)
-end
-
 --[[
 	Relationship pairs, just like regular component, can be associated with data.
 ]]
@@ -178,30 +122,4 @@ for entity, eats_data in world:query(pair(Eats, Apples)) do
 	print(`Entity {entity} eats apples: amount = {eats_data.amount}`)
 end
 
---[[
-	When querying for relationship pairs, it is often useful to be able to find
-	all instances for a given relationship or target. To accomplish this, a game
-	can use wildcard expressions.
-
-	Wildcards may used for the relationship or target part of a pair:
-
-	pair(Likes, jecs.Wildcard) -- Matches all Likes relationships
-	pair(jecs.Wildcard, Alice) -- Matches all relationships with Alice as target
-
-	Using world:target() is the recommended way to get the target in a wildcard
-	query. However, if you're in a very hot path and need maximum performance,
-	you can access the relationship column directly (see advanced examples).
-]]
-
-for entity in world:query(pair(Eats, jecs.Wildcard)) do
-	local nth = 0
-	local food = world:target(entity, Eats, nth)
-	while food do
-		local eats_data = world:get(entity, pair(Eats, food))
-		assert(eats_data) -- This coerces the type to be non-nilable for the type checker
-		print(`Entity {entity} eats {food}: amount = {eats_data.amount}`)
-
-		nth += 1
-		food = world:target(entity, Eats, nth)
-	end
-end
+-- For wildcard queries and world:target (0-based index), see 042_target.luau and 043_wildcards.luau.

how_to/041_entity_relationships.luau

@@ -0,0 +1,37 @@
+--[[
+	world:target(entity, relation, index?) returns the target of a relationship
+	on an entity. The index is 0-based. If no target exists at that index, it
+	returns nil.
+
+	Use target when you have queried with a wildcard (e.g. pair(Eats, jecs.Wildcard))
+	and need the actual target entity for each result. Without an index, the
+	default is 0 (the first target).
+]]
+
+local jecs = require("@jecs")
+local pair = jecs.pair
+local world = jecs.world()
+
+local Eats = world:entity()
+local Apples = world:entity()
+local Oranges = world:entity()
+local bob = world:entity()
+
+world:add(bob, pair(Eats, Apples))
+world:add(bob, pair(Eats, Oranges))
+
+-- First target is at index 0
+local first = world:target(bob, Eats, 0)
+print(first == Apples)  -- true
+
+-- Second target is at index 1
+local second = world:target(bob, Eats, 1)
+print(second == Oranges)  -- true
+
+-- No third target: index 2 returns nil
+local third = world:target(bob, Eats, 2)
+print(third == nil)  -- true
+
+-- Omitting the index is the same as index 0
+local default = world:target(bob, Eats)
+print(default == Apples)  -- true

how_to/042_target.luau

@@ -0,0 +1,58 @@
+--[[
+	Wildcards let you query relationships without specifying the exact target or
+	relationship. jecs.Wildcard matches any entity in that slot.
+
+	- pair(relation, jecs.Wildcard) matches that relationship with any target.
+	- pair(jecs.Wildcard, target) matches any relationship with that target.
+
+	Use world:target(entity, relation, index) to get the actual target when
+	querying with a wildcard. The index is 0-based (see 042_target.luau).
+]]
+
+local jecs = require("@jecs")
+local pair = jecs.pair
+local world = jecs.world()
+
+local Eats = world:component() :: jecs.Id<{ amount: number }>
+local Likes = world:entity()
+local Apples = world:entity()
+local alice = world:entity()
+local bob = world:entity()
+
+world:add(bob, pair(Eats, Apples))
+world:set(bob, pair(Eats, Apples), { amount = 1 })
+world:add(bob, pair(Likes, alice))
+
+-- world:has with wildcard
+print(world:has(bob, pair(Eats, jecs.Wildcard)))  -- true
+
+-- Query with wildcard: all entities that eat something
+for entity in world:query(pair(Eats, jecs.Wildcard)) do
+	local food = world:target(entity, Eats)
+	print(`Entity {entity} eats {food}`)
+end
+
+-- Query with wildcard: all entities that like someone
+for entity in world:query(pair(Likes, jecs.Wildcard)) do
+	local target = world:target(entity, Likes)
+	print(`Entity {entity} likes {target}`)
+end
+
+-- Multiple targets: index is 0-based. Iterate until nil.
+local charlie = world:entity()
+world:add(bob, pair(Likes, charlie))
+
+for entity in world:query(pair(Likes, jecs.Wildcard)) do
+	local nth = 0
+	local target = world:target(entity, Likes, nth)
+	while target do
+		print(`Entity {entity} likes {target}`)
+		nth += 1
+		target = world:target(entity, Likes, nth)
+	end
+end
+
+-- pair(jecs.Wildcard, target): all relationships that have this target
+for entity in world:query(pair(jecs.Wildcard, alice)) do
+	print(`Entity {entity} has some relationship with alice`)
+end