Make sure equations returns Vector{Equation}

Fix #716

Author: YingboMa

src/systems/abstractsystem.jl

@@ -176,7 +176,11 @@ function namespace_pins(sys::AbstractSystem)
     [renamespace(sys.name,x) for x in pins(sys)]
 end
 
-namespace_equations(sys::AbstractSystem) = namespace_equation.(equations(sys),sys.name,sys.iv.name)
+function namespace_equations(sys::AbstractSystem)
+    eqs = equations(sys)
+    isempty(eqs) && return Equation[]
+    map(eq->namespace_equation(eq,sys.name,sys.iv.name), eqs)
+end
 
 function namespace_equation(eq::Equation,name,ivname)
     _lhs = namespace_expr(eq.lhs,name,ivname)
@@ -236,11 +240,10 @@ function equations(sys::ModelingToolkit.AbstractSystem)
     if isempty(sys.systems)
         return sys.eqs
     else
-        eqs = [sys.eqs;
+        eqs = Equation[sys.eqs;
                reduce(vcat,
                       namespace_equations.(sys.systems);
                       init=Equation[])]
-
         return eqs
     end
 end

test/reduction.jl

@@ -114,3 +114,32 @@ let
     test_equal.(asys.eqs, [D(x) ~ 2x])
     test_equal.(asys.observed, [y ~ x])
 end
+
+# issue #716
+let
+    @parameters t
+    @derivatives D'~t
+
+    ## Define a system
+
+    @variables x(t), u(t), y(t)
+    @parameters a, b, c, d
+    ol = ODESystem([D(x) ~ a * x + b * u, y ~ c * x], t, name=:ol)
+
+    ## Define another system that is purely observing
+
+    @variables u_c(t), y_c(t)
+    @parameters k_P
+    pc = ODESystem(Equation[], t, pins=[y_c], observed = [u_c ~ k_P * y_c], name=:pc)
+
+    ## Connect them
+
+    connections = [
+                   ol.u ~ pc.u_c,
+                   y_c ~ ol.y
+                  ]
+
+    connected = MTK.ODESystem(connections, t, systems=[ol, pc])
+
+    @test_nowarn MTK.flatten(connected)
+end