Provide completions for expressions within the data mapper

flow-model-generator/modules/flow-model-generator-core/src/main/java/io/ballerina/flowmodelgenerator/core/expressioneditor/ExpressionEditorContext.java

@@ -207,6 +207,45 @@ public LineRange generateStatement() {
         return statementLineRange;
     }
 
+    /**
+     * Generates a Ballerina function definition with the provided signature, containing the statement
+     * generated by {@link #generateStatement()} as the function body.
+     *
+     * <p>
+     * TODO: This is a temporary workaround to provide completions for the data mapper, and should be removed once
+     * the API extension is complete.
+     *
+     * @param functionSignature the function signature (e.g., "(Person person, Admission admission) returns Employee")
+     * @return the line range of the generated function definition
+     */
+    public LineRange generateFunctionDefinition(String functionSignature) {
+        // Get the text position of the start line
+        TextDocument textDocument = documentContext.document().textDocument();
+        LinePosition cursorStartLine = startLine();
+        int textPosition = textDocument.textPositionFrom(cursorStartLine);
+
+        // Build function header with opening brace and indentation
+        String functionHeader = String.format("function %s {%n    ", functionSignature);
+
+        // Apply the function header
+        List<TextEdit> textEdits = new ArrayList<>();
+        textEdits.add(TextEdit.from(TextRange.from(textPosition, 0), functionHeader));
+        applyTextEdits(textEdits);
+
+        // Calculate the number of lines added by the function header
+        int functionHeaderLines = functionHeader.split(System.lineSeparator()).length - 1;
+
+        // Update the start line to account for the function header
+        // Store original start line for later calculation
+        this.startLine = LinePosition.from(
+            cursorStartLine.line() + functionHeaderLines,
+            4  // Indentation of 4 spaces
+        );
+
+        // Generate the statement inside the function body
+        return generateStatement();
+    }
+
     public LineRange getExpressionLineRange() {
         LinePosition startLine = info().startLine();
         LinePosition endLine =

flow-model-generator/modules/flow-model-generator-core/src/main/java/io/ballerina/flowmodelgenerator/core/expressioneditor/services/CompletionRequest.java

@@ -18,8 +18,15 @@
 
 package io.ballerina.flowmodelgenerator.core.expressioneditor.services;
 
+import com.google.gson.Gson;
+import com.google.gson.JsonElement;
+import io.ballerina.compiler.syntax.tree.FunctionDefinitionNode;
+import io.ballerina.compiler.syntax.tree.NonTerminalNode;
 import io.ballerina.flowmodelgenerator.core.expressioneditor.ExpressionEditorContext;
 import io.ballerina.flowmodelgenerator.core.model.Property;
+import io.ballerina.tools.text.LineRange;
+import org.ballerinalang.langserver.common.utils.CommonUtil;
+import org.ballerinalang.langserver.common.utils.PositionUtil;
 import org.eclipse.lsp4j.CompletionContext;
 import org.eclipse.lsp4j.CompletionItem;
 import org.eclipse.lsp4j.CompletionItemKind;
@@ -45,6 +52,7 @@ public class CompletionRequest extends DebouncedExpressionEditorRequest<Either<L
     private final CompletionContext completionContext;
     private final TextDocumentService textDocumentService;
     private static final String RESERVED_VARIABLE_NAME = "__reserved__";
+    private static final Gson GSON = new Gson();
 
     public CompletionRequest(ExpressionEditorContext context, CompletionContext completionContext,
                              TextDocumentService textDocumentService) {
@@ -55,7 +63,28 @@ public CompletionRequest(ExpressionEditorContext context, CompletionContext comp
 
     @Override
     public Either<List<CompletionItem>, CompletionList> getResponse(ExpressionEditorContext context) {
-        context.generateStatement();
+        String valueType = context.getProperty().valueType();
+
+        // TODO: Added a special case for expressions used within the data mapper, as inputs are not provided through
+        //  the traditional method.
+        boolean generatedFunctionDefinition = false;
+        if (Property.ValueType.DATA_MAPPING_EXPRESSION.name().equals(valueType)) {
+            JsonElement lineRangeJson = context.info().codedata().get("lineRange");
+            LineRange lineRange = GSON.fromJson(lineRangeJson, LineRange.class);
+
+            if (lineRange != null) {
+                NonTerminalNode node = CommonUtil.findNode(PositionUtil.toRange(lineRange),
+                        context.documentContext().document().syntaxTree());
+                if (node instanceof FunctionDefinitionNode functionDefinitionNode) {
+                    context.generateFunctionDefinition(functionDefinitionNode.functionSignature().toString());
+                    generatedFunctionDefinition = true;
+                }
+            }
+        }
+        if (!generatedFunctionDefinition) {
+            context.generateStatement();
+        }
+
         Position position = context.getCursorPosition();
         TextDocumentIdentifier identifier = new TextDocumentIdentifier(context.fileUri());
         CompletionParams params = new CompletionParams(identifier, position, completionContext);
@@ -67,7 +96,6 @@ public Either<List<CompletionItem>, CompletionList> getResponse(ExpressionEditor
 
         // Filter the completions if it is a lvexpr
         // TODO: Extend the implementation to a different class
-        String valueType = context.getProperty().valueType();
         if (Property.ValueType.LV_EXPRESSION.name().equals(valueType)) {
             List<CompletionItem> completionsList;
             if (completions.getLeft() != null) {

flow-model-generator/modules/flow-model-generator-core/src/main/java/io/ballerina/flowmodelgenerator/core/model/Property.java

@@ -265,7 +265,8 @@ public enum ValueType {
         FIXED_PROPERTY,
         RAW_TEMPLATE,
         REPEATABLE_PROPERTY,
-        ACTION_PATH
+        ACTION_PATH,
+        DATA_MAPPING_EXPRESSION
     }
 
     public static ValueType valueTypeFrom(String s) {