Merge pull request #122 from SciML/b/fix-deim-complete-system

Fix DEIM system construction by completing system before return

Author: bowenszhu

docs/src/tutorials/deim_FitzHugh-Nagumo.md

@@ -181,7 +181,7 @@ plt_2 = plot(xlabel = L"v(x,t)", ylabel = L"x", zlabel = L"w(x,t)", xlims = (-0.
     ylims = (0.0, L), zlims = (0.0, 0.25), xflip = true, camera = (50, 30),
     titlefont = 10, title = "Comparison of full and reduced systems")
 plot!(plt_2, unconnected(snapshot_v), unconnected(sol_x, nₜ), unconnected(snapshot_w),
-    label = "Full$(length(ode_sys.eqs))")
+    label = "Full$(length(ModelingToolkit.get_eqs(ode_sys)))")
 plot!(plt_2, unconnected(sol_deim_v), unconnected(sol_deim_x, nₜ_deim),
     unconnected(sol_deim_w), label = "POD$(pod_dim)/DEIM$(deim_dim)")
 ```

src/deim.jl

@@ -172,4 +172,11 @@ function deim(sys::ODESystem, snapshot::AbstractMatrix, pod_dim::Integer;
 
     inv_dict = Dict(Symbolics.scalarize(ŷ .=> V' * dvs)) # reduced vars to original vars
     @set! sys.defaults = merge(ModelingToolkit.defaults(sys), inv_dict)
+
+    # CRITICAL: Call complete() on the system before returning to ensure all subsystems,
+    # variables, and parameters are properly registered and namespaced. Without this, 
+    # attempting to create an ODEProblem from the DEIM system will fail with errors about
+    # missing initial conditions for variables that should exist in the system.
+    # This is required due to changes in ModelingToolkit.jl's internal structure handling.
+    return complete(sys)
 end