Implement substitute for AbstractSystems

src/systems/abstractsystem.jl

@@ -1831,3 +1831,21 @@ function missing_variable_defaults(sys::AbstractSystem, default = 0.0)
 
     return ds
 end
+
+keytype(::Type{<:Pair{T, V}}) where {T, V} = T
+function Symbolics.substitute(sys::AbstractSystem, rules::Union{Vector{<:Pair}, Dict})
+    if keytype(eltype(rules)) <: Symbol
+        dict = todict(rules)
+        systems = get_systems(sys)
+        # post-walk to avoid infinite recursion
+        @set! sys.systems = map(Base.Fix2(substitute, dict), systems)
+        something(get(rules, nameof(sys), nothing), sys)
+    elseif sys isa ODESystem
+        rules = todict(map(r -> Symbolics.unwrap(r[1]) => Symbolics.unwrap(r[2]),
+            collect(rules)))
+        eqs = fast_substitute(equations(sys), rules)
+        ODESystem(eqs, get_iv(sys); name = nameof(sys))
+    else
+        error("substituting symbols is not supported for $(typeof(sys))")
+    end
+end

src/systems/diffeqs/odesystem.jl

@@ -465,13 +465,6 @@ function convert_system(::Type{<:ODESystem}, sys, t; name = nameof(sys))
         checks = false)
 end
 
-function Symbolics.substitute(sys::ODESystem, rules::Union{Vector{<:Pair}, Dict})
-    rules = todict(map(r -> Symbolics.unwrap(r[1]) => Symbolics.unwrap(r[2]),
-        collect(rules)))
-    eqs = fast_substitute(equations(sys), rules)
-    ODESystem(eqs, get_iv(sys); name = nameof(sys))
-end
-
 """
 $(SIGNATURES)
 

test/odesystem.jl

@@ -1012,3 +1012,22 @@ let
     prob = ODAEProblem(sys4s, [x => 1.0, D(x) => 1.0], (0, 1.0))
     @test !isnothing(prob.f.sys)
 end
+
+@parameters t
+# SYS 1:
+vars_sub1 = @variables s1(t)
+@named sub = ODESystem(Equation[], t, vars_sub1, [])
+
+vars1 = @variables x1(t)
+@named sys1 = ODESystem(Equation[], t, vars1, [], systems = [sub])
+@named sys2 = ODESystem(Equation[], t, vars1, [], systems = [sys1, sub])
+
+# SYS 2: Extension to SYS 1
+vars_sub2 = @variables s2(t)
+@named partial_sub = ODESystem(Equation[], t, vars_sub2, [])
+@named sub = extend(partial_sub, sub)
+
+new_sys2 = complete(substitute(sys2, Dict(:sub => sub)))
+Set(states(new_sys2)) == Set([new_sys2.x1, new_sys2.sys1.x1,
+    new_sys2.sys1.sub.s1, new_sys2.sys1.sub.s2,
+    new_sys2.sub.s1, new_sys2.sub.s2])