Fix Liquid expressions not working in sub-workflows (workflow-as-activity) (#6678)

* Initial plan for issue

* Fix Liquid expressions not working in sub-workflows

Co-authored-by: sfmskywalker <938393+sfmskywalker@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: sfmskywalker <938393+sfmskywalker@users.noreply.github.com>

src/modules/Elsa.Expressions.Liquid/Handlers/ConfigureLiquidEngine.cs

@@ -75,10 +75,18 @@ internal class ConfigureLiquidEngine : INotificationHandler<RenderingLiquidTempl
 
     private Task<FluidValue> GetInput(ExpressionExecutionContext context, string key, TemplateOptions options)
     {
-        var workflowExecutionContext = context.GetWorkflowExecutionContext();
-        var input = workflowExecutionContext.Input.TryGetValue(key, out var value) ? value : default;
+        // First, check if the current activity has inputs
+        if (context.TryGetActivityExecutionContext(out var activityExecutionContext) &&
+            activityExecutionContext.Input.TryGetValue(key, out var activityValue))
+        {
+            return Task.FromResult(activityValue == null ? NilValue.Instance : FluidValue.Create(activityValue, options));
+        }
         
-        return Task.FromResult(input == null ? NilValue.Instance : FluidValue.Create(value, options));
+        // Fall back to workflow inputs if activity inputs don't contain the key
+        var workflowExecutionContext = context.GetWorkflowExecutionContext();
+        var input = workflowExecutionContext.Input.TryGetValue(key, out var workflowValue) ? workflowValue : default;
+        
+        return Task.FromResult(input == null ? NilValue.Instance : FluidValue.Create(workflowValue, options));
     }
 
     private static object? GetVariableInScope(ExpressionExecutionContext context, string variableName)

test/integration/Elsa.Workflows.IntegrationTests/Scenarios/LiquidSubWorkflow/LiquidSubWorkflowTests.cs

@@ -0,0 +1,40 @@
+using Elsa.Testing.Shared;
+using Elsa.Workflows.Runtime;
+using Microsoft.Extensions.DependencyInjection;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace Elsa.Workflows.IntegrationTests.Scenarios.LiquidSubWorkflow;
+
+public class LiquidSubWorkflowTests
+{
+    private readonly ITestOutputHelper _testOutputHelper;
+
+    public LiquidSubWorkflowTests(ITestOutputHelper testOutputHelper)
+    {
+        _testOutputHelper = testOutputHelper;
+    }
+    
+    [Fact]
+    public async Task ShouldBeAbleToUseLiquidExpressionsToReadInputInSubWorkflows()
+    {
+        // Arrange.
+        var services = new TestApplicationBuilder()
+            .WithCapturingTextWriter()
+            .Build();
+
+        var workflowRunner = services.GetRequiredService<IWorkflowRunner>();
+        
+        // Act.
+        await workflowRunner.RunAsync<LiquidParentWorkflow>();
+
+        // Assert.
+        var capturedOutput = services.GetRequiredService<StringWriter>();
+        var output = capturedOutput.ToString();
+        _testOutputHelper.WriteLine(output);
+
+        // Verify that the liquid expressions were able to read the input values.
+        Assert.Contains("Person: John Doe, Email: john@example.com", output);
+        Assert.Contains("Sub workflow result: John Doe - john@example.com", output);
+    }
+}

test/integration/Elsa.Workflows.IntegrationTests/Scenarios/LiquidSubWorkflow/Workflows.cs

@@ -0,0 +1,76 @@
+using Elsa.Extensions;
+using Elsa.Workflows.Activities;
+using Elsa.Workflows.Management.Activities.SetOutput;
+using Elsa.Workflows.Memory;
+using Elsa.Workflows.Models;
+using Elsa.Expressions.Models;
+
+namespace Elsa.Workflows.IntegrationTests.Scenarios.LiquidSubWorkflow;
+
+/// <summary>
+/// A sub-workflow that reads input using liquid expressions
+/// </summary>
+public class LiquidSubWorkflow : WorkflowBase
+{
+    public Input<string> PersonName { get; set; } = default!;
+    public Input<string> PersonEmail { get; set; } = default!;
+    public Output<string> Result { get; set; } = default!;
+
+    protected override void Build(IWorkflowBuilder builder)
+    {
+        var resultVariable = new Variable<string>("Result");
+        
+        builder.Root = new Sequence
+        {
+            Variables = { resultVariable },
+            Activities =
+            {
+                // Use liquid to read inputs
+                new WriteLine(new Expression("Liquid", "Person: {{ Input.PersonName }}, Email: {{ Input.PersonEmail }}")),
+
+                // Set the result with liquid expression
+                new SetVariable
+                {
+                    Variable = resultVariable,
+                    Value = new Expression("Liquid", "{{ Input.PersonName }} - {{ Input.PersonEmail }}")
+                },
+                
+                // Set output
+                new SetOutput
+                {
+                    OutputName = new("Result"),
+                    OutputValue = new(resultVariable) 
+                }
+            }
+        };
+    }
+}
+
+/// <summary>
+/// A main workflow that uses the sub-workflow as an activity
+/// </summary>
+public class LiquidParentWorkflow : WorkflowBase
+{
+    protected override void Build(IWorkflowBuilder builder)
+    {
+        var resultVariable = new Variable<string>("Result");
+        
+        var subWorkflow = new LiquidSubWorkflow
+        {
+            PersonName = new("John Doe"),
+            PersonEmail = new("john@example.com"),
+            Result = new(resultVariable)
+        };
+
+        builder.Variables.Add(resultVariable);
+
+        builder.Root = new Sequence
+        {
+            Activities =
+            {
+                subWorkflow,
+                new WriteLine(context => $"Sub workflow result: {resultVariable.Get(context)}")
+            }
+        };
+    }
+}