modify-typed-resource

name	modify-typed-resource
description	[AzureRM-specific] Assist in adding a new property to an existing legacy resource (Typed SDK wrapper).
triggers	["add property to typed resource","modify typed sdk resource","internal/sdk/resource.go modification"]

[AzureRM-specific] Modify Typed Resource Skill

This skill assists in adding a new property to an existing legacy resource that uses the "Typed" SDK wrapper (internal/sdk/resource.go).

Identification

A typed resource implements the sdk.Resource interface and typically has a companion *Model struct.

type ExampleResourceModel struct {
    // ...
}

type ExampleResource struct{}

Atomicity & CRUD Methods

[!CAUTION] Remediate Combined Create/Update. If the existing Typed resource currently uses the same Func for both Create and Update, or shares a helper function (like submit or createUpdate) between them, you MUST split the logic into discrete, independent Create and Update methods as part of your modification. Each CRUD method must manage its own client calls and payload construction independently. The Typed wrapper SDK automatically handles read-after-create and read-after-update, so the Func should simply return nil upon success rather than calling the Read function.

Steps to Add a Property

1. Update the Model Struct

Add the field to the model struct with the tfschema tag.

type ExampleResourceModel struct {
    // ...
    NewProperty string `tfschema:"new_property"`
}

2. Update Arguments or Attributes

Add the field to either the Arguments() (if user-configurable) or Attributes() (if read-only) method.

func (r ExampleResource) Arguments() map[string]*pluginsdk.Schema {
    return map[string]*pluginsdk.Schema{
        // ...
        "new_property": {
            Type:     pluginsdk.TypeString,
            Optional: true,
        },
    }
}

3. Update Create Function

Access the property directly from the model after decoding.

func (r ExampleResource) Create() sdk.ResourceFunc {
    return sdk.ResourceFunc{
        Func: func(ctx context.Context, metadata sdk.ResourceMetaData) error {
            var model ExampleResourceModel
            if err := metadata.Decode(&model); err != nil {
                return err
            }
            // Use model.NewProperty
        },
    }
}

4. Update the Update Function

In the Update() method, check if the property has changed before including it in the update payload (especially for PATCH operations).

func (r ExampleResource) Update() sdk.ResourceFunc {
    return sdk.ResourceFunc{
        Func: func(ctx context.Context, metadata sdk.ResourceMetaData) error {
            // ...
            if metadata.ResourceData.HasChange("new_property") {
                // include in patch/update payload
            }
        },
    }
}

5. Update the Read Function

Set the property in the state model before encoding.

func (r ExampleResource) Read() sdk.ResourceFunc {
    return sdk.ResourceFunc{
        Func: func(ctx context.Context, metadata sdk.ResourceMetaData) error {
            // ...
            state.NewProperty = *apiResponse.NewProperty
            return metadata.Encode(&state)
        },
    }
}

Adding CustomizeDiff

To implement CustomizeDiff on a Typed resource, the resource struct must implement the sdk.ResourceWithCustomizeDiff interface. Add the compliance assertion and the method:

// Compile-time assertion — add alongside other interface assertions at the top of the file
var _ sdk.ResourceWithCustomizeDiff = ExampleResource{}

func (r ExampleResource) CustomizeDiff() sdk.ResourceFunc {
    return sdk.ResourceFunc{
        Timeout: 5 * time.Minute,
        Func: func(ctx context.Context, metadata sdk.ResourceMetaData) error {
            if metadata.ResourceDiff == nil {
                return nil
            }

            var model ExampleResourceModel
            if err := metadata.DecodeDiff(&model); err != nil {
                return err
            }

            // logic here

            return nil
        },
    }
}

The SDK wrapper in sdk/wrapper_resource.go automatically detects the interface and wires up CustomizeDiff — no changes to the resource registration function are needed.

Safety & Verification

• Human Review: Verify that the tfschema tag matches the schema key.
• Model Validation: Ensure the model struct accurately reflects the API response.
• State Awareness: Use terraform plan to verify that the new property is correctly tracked in the state.