implement-datasource

name	implement-datasource
description	How to add or modify a Terraform data source in this provider. Covers scaffolding, unknown input handling, list data sources with pagination, and API response validation.

Implement Data Source

Step-by-step guide for adding a new data source or modifying an existing one. Before starting, also read the implementation-conventions skill for shared patterns.

Step 1: Add to datasources.yml

Add the data source definition to OpenAPI/1_tfplugingen-openapi/datasources.yml:

data_sources:
  - name: my_resource
    schema:
      attributes:
        overrides:
          - name: id
            data_source:
              attribute_type:
                name: string
                optional: false # Make ID required input

Step 2: Generate Code

make generate

This generates the schema and model types in internal/provider/datasource_<name>/.

Step 3: Scaffold the Data Source

Always use the scaffold command — do NOT write from scratch:

go run github.com/doitintl/terraform-plugin-codegen-framework/cmd/tfplugingen-framework scaffold data-source \
  --name <name> \
  --output-file <name>_data_source.go \
  --package provider \
  --output-dir internal/provider

NOTE: --output-file must be the basename only (e.g., my_data_source.go), not a full path.

Required interface declarations (verify in scaffolded file):

var _ datasource.DataSource = (*myDataSource)(nil)
var _ datasource.DataSourceWithConfigure = (*myDataSource)(nil)

Step 4: Implement the Data Source

Mapping Convention

Data sources that share an entity with a resource (e.g. alert, report) should reuse the mapXxxToModel helper from the companion <name>.go file. Data sources with their own unique mapping (e.g. list data sources) should place helpers in the _data_source.go file or in a companion file when the mapping is non-trivial.

Simple data sources that only do a few scalar assignments may keep mapping logic inline in Read.

Implementation

• Use *models.ClientWithResponses for client type
• Use generated constructors (NewXxxValue()) for nested objects
• Add timeout support (see implementation-conventions)

Unknown Input Handling (Critical)

Data sources are read during terraform plan. If an input depends on an unresolved resource, its value will be Unknown. Check for unknown inputs before making API calls:

Scalar inputs — check with IsUnknown():

if data.Id.IsUnknown() {
    data.Result = types.StringUnknown()
    resp.Diagnostics.Append(resp.State.Set(ctx, &data)...)
    return
}

Composite inputs (lists, maps, objects) — use IsFullyKnown():

if !req.Config.Raw.IsFullyKnown() {
    data.Id = types.StringUnknown()
    data.Results = types.ListUnknown(elemType)
    resp.Diagnostics.Append(resp.State.Set(ctx, &data)...)
    return
}

A list can be known while containing unknown elements (e.g., resources = [some_resource.id]), which IsUnknown() on the list itself would miss.

IMPORTANT: Return Unknown, not Null. Null means "this value does not exist" while Unknown means "not yet determined." Downstream consumers treat Null as a real value during planning.

Linter: unknownguard — flags data source Read methods missing unknown input checks.

API Response Validation

Validate both status code and parsed response body:

if apiResp.StatusCode() != 200 || apiResp.JSON200 == nil {
    resp.Diagnostics.AddError(
        "Error Reading Resource",
        fmt.Sprintf("status: %d, body: %s", apiResp.StatusCode(), string(apiResp.Body)),
    )
    return
}

The generated client only populates JSON200 when the response Content-Type is JSON.

Computed-Only List Attributes

Always return an empty list [] instead of null for computed-only list attributes. Returning null causes errors when consumers iterate (e.g., for_each, for expressions):

// CORRECT — empty list is safely iterable
emptyList, diags := types.ListValueFrom(ctx, elemType, []MyValueType{})
data.Items = emptyList

// WRONG — causes "Iteration over null value" errors
data.Items = types.ListNull(elemType)

Step 5: Register the Data Source

Add to provider.go in the DataSources() method.

Step 6: Add Tests and Examples

• Test file: internal/provider/<name>_data_source_test.go
• Example: examples/data-sources/doit_<name>/data-source.tf
• Add test env vars to .envrc.local
• Always terraform apply every example at least once to verify

Step 7: Generate Docs

make docs

---

List Data Sources (Plural Endpoints)

List data sources (e.g., doit_budgets, doit_allocations) return arrays of items.

Smart Pagination

List data sources support smart pagination — they honor user-provided values when set, otherwise auto-paginate:

User Sets	Behavior
Neither max_results nor page_token	Auto-paginate: fetch all pages
max_results only	Single page: return up to N items
page_token only	Auto-paginate from token
Both	Manual pagination

userControlsPagination := !data.MaxResults.IsNull() && !data.MaxResults.IsUnknown()

if userControlsPagination {
    params.MaxResults = new(data.MaxResults.ValueString())
    // Single API call, preserve page_token from response
} else {
    // Auto mode: fetch all pages
    var allItems []models.ItemType
    for {
        apiResp, _ := d.client.ListItemsWithResponse(ctx, params)
        allItems = append(allItems, *apiResp.JSON200.Items...)
        if apiResp.JSON200.PageToken == nil || *apiResp.JSON200.PageToken == "" {
            break
        }
        params.PageToken = apiResp.JSON200.PageToken
    }
}

Key principle: User-provided values MUST be preserved in state to prevent perpetual diffs.

Mapping API Types to Generated Values

Use NewXxxValue() constructors with map[string]attr.Value{}:

budgetVal, diags := datasource_budgets.NewBudgetsValue(
    datasource_budgets.BudgetsValue{}.AttributeTypes(ctx),
    map[string]attr.Value{
        "id":          types.StringPointerValue(budget.Id),
        "budget_name": types.StringPointerValue(budget.BudgetName),
        "amount":      types.Float64PointerValue(budget.Amount),
    },
)

Type Handling Tips

1. Pointer vs Non-Pointer: Check models_gen.go — pointer (*string) → types.StringPointerValue(), non-pointer → types.StringValue()
2. Nullable fields: nullable.Nullable[T] → wrap with nullableToPointer() before passing to types.XxxPointerValue(). Example: types.StringPointerValue(nullableToPointer(resp.Field))
3. Enum Types: Convert to string first — types.StringValue(string(s.Mode))
4. Nested Lists: Create helper functions to map nested structures

RowCount Determinism

Always guarantee a deterministic fallback by calculating slice length when the API omits rowCount:

if apiResp.JSON200.RowCount != nil {
    data.RowCount = types.Int64Value(*apiResp.JSON200.RowCount)
} else {
    data.RowCount = types.Int64Value(int64(len(items)))
}

Common Mistake

Generated schema field names match the API's JSON tags in snake_case, NOT arbitrary Terraform conventions. Verify names against datasource_xxx/xxx_data_source_gen.go and models/models_gen.go.