add test file

test/test_world.jl

@@ -0,0 +1,49 @@
+using Multibody, ModelingToolkit, JuliaSimCompiler, Test
+@mtkmodel FallingBody begin
+ @components begin
+ my_world = World(g = 1, n = [0, 1, 0])
+ body = Body(isroot=true)
+ end
+end
+
+@named model = FallingBody()
+model = complete(model)
+ssys = structural_simplify(IRSystem(model))
+prob = ODEProblem(ssys, [], (0, 1))
+sol = solve(prob, Tsit5())
+
+tv = 0:0.1:1
+@test iszero(sol(tv, idxs=model.body.r_0[1]))
+@test sol(tv, idxs=model.body.r_0[2]) ≈ tv.^2 ./ 2 atol=1e-6
+@test iszero(sol(tv, idxs=model.body.r_0[3]))
+
+
+# Change g
+prob = ODEProblem(ssys, [model.my_world.g .=> 2], (0, 1))
+sol = solve(prob, Tsit5())
+@test iszero(sol(tv, idxs=model.body.r_0[1]))
+@test sol(tv, idxs=model.body.r_0[2]) ≈ 2*tv.^2 ./ 2 atol=1e-6
+@test iszero(sol(tv, idxs=model.body.r_0[3]))
+
+# Change n
+prob = ODEProblem(ssys, collect(model.my_world.n) .=> [1, 0, 0], (0, 1))
+sol = solve(prob, Tsit5())
+@test sol(tv, idxs=model.body.r_0[1]) ≈ tv.^2 ./ 2 atol=1e-6
+@test iszero(sol(tv, idxs=model.body.r_0[2]))
+@test iszero(sol(tv, idxs=model.body.r_0[3]))
+
+
+## World in more than one place
+# This should result in too many equations
+@mtkmodel FallingBodyOuter begin
+ @components begin
+ inner_model = FallingBody()
+ my_world_outer = World(g = 2, n = [0, 1, 0])
+ body = Body(isroot=true)
+ end
+end
+
+@named model = FallingBodyOuter()
+model = complete(model)
+@test_throws ModelingToolkit.ExtraEquationsSystemException structural_simplify(IRSystem(model))
+