Allow falling back to single successor meters in grid formulas

Signed-off-by: Sahas Subramanian <[email protected]>

Author: shsms

src/graph/formulas/generators/grid.rs

@@ -35,7 +35,7 @@ where
         for comp in self.graph.successors(self.graph.root_id)? {
             let comp = self
                 .graph
-                .fallback_expr([comp.component_id()], true, false)?;
+                .fallback_expr([comp.component_id()], true, true)?;
             expr = match expr {
                 None => Some(comp),
                 Some(e) => Some(comp + e),
@@ -59,14 +59,19 @@ mod tests {
 
         // Add a grid meter and a battery chain behind it.
         let grid_meter = builder.meter();
-        let meter_bat_chain = builder.meter_bat_chain(1, 1);
         builder.connect(grid, grid_meter);
-        builder.connect(grid_meter, meter_bat_chain);
 
         let graph = builder.build(None)?;
         let formula = graph.grid_formula()?.to_string();
         assert_eq!(formula, "#1");
 
+        let meter_bat_chain = builder.meter_bat_chain(1, 1);
+        builder.connect(grid_meter, meter_bat_chain);
+
+        let graph = builder.build(None)?;
+        let formula = graph.grid_formula()?.to_string();
+        assert_eq!(formula, "COALESCE(#1, #2)");
+
         // Add an additional dangling meter, and a PV chain and a battery chain
         // to the grid
         let dangling_meter = builder.meter();
@@ -84,7 +89,7 @@ mod tests {
         let formula = graph.grid_formula()?.to_string();
         assert_eq!(
             formula,
-            "#1 + #5 + COALESCE(#6, #7, 0.0) + COALESCE(#9, #10, 0.0)"
+            "COALESCE(#1, #2) + #5 + COALESCE(#6, #7, 0.0) + COALESCE(#9, #10, 0.0)"
         );
 
         // Add a PV inverter to the grid, without a meter.
@@ -97,7 +102,7 @@ mod tests {
         let formula = graph.grid_formula()?.to_string();
         assert_eq!(
             formula,
-            "#1 + #5 + COALESCE(#6, #7, 0.0) + COALESCE(#9, #10, 0.0) + COALESCE(#11, 0.0)"
+            "COALESCE(#1, #2) + #5 + COALESCE(#6, #7, 0.0) + COALESCE(#9, #10, 0.0) + COALESCE(#11, 0.0)"
         );
 
         Ok(())