test: Ensures project withDefaultAlertsSettings works with import and introduce create_only plan modifier

.changelog/3105.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+resource/mongodbatlas_project: Fixes import when `with_default_alerts_settings` is set 
+```

internal/common/customplanmodifier/create_only.go

@@ -0,0 +1,52 @@
+package customplanmodifier
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/hashicorp/terraform-plugin-framework/attr"
+	"github.com/hashicorp/terraform-plugin-framework/diag"
+	"github.com/hashicorp/terraform-plugin-framework/path"
+	planmodifier "github.com/hashicorp/terraform-plugin-framework/resource/schema/planmodifier"
+)
+
+type Modifier interface {
+	planmodifier.String
+	planmodifier.Bool
+}
+
+func CreateOnlyAttributePlanModifier() Modifier {
+	return &createOnlyAttributePlanModifier{}
+}
+
+type createOnlyAttributePlanModifier struct {
+}
+
+func (d *createOnlyAttributePlanModifier) Description(ctx context.Context) string {
+	return d.MarkdownDescription(ctx)
+}
+
+func (d *createOnlyAttributePlanModifier) MarkdownDescription(ctx context.Context) string {
+	return "Ensures that update operations fails when updating an attribute."
+}
+
+func (d *createOnlyAttributePlanModifier) PlanModifyBool(ctx context.Context, req planmodifier.BoolRequest, resp *planmodifier.BoolResponse) {
+	if d.isUpdated(req.PlanValue, req.StateValue) {
+		d.addDiags(&resp.Diagnostics, req.Path)
+	}
+}
+
+func (d *createOnlyAttributePlanModifier) PlanModifyString(ctx context.Context, req planmodifier.StringRequest, resp *planmodifier.StringResponse) {
+	if d.isUpdated(req.PlanValue, req.StateValue) {
+		d.addDiags(&resp.Diagnostics, req.Path)
+	}
+}
+
+func (d *createOnlyAttributePlanModifier) isUpdated(planValue, stateValue attr.Value) bool {
+	return !stateValue.IsNull() && !planValue.Equal(stateValue)
+}
+
+func (d *createOnlyAttributePlanModifier) addDiags(diags *diag.Diagnostics, attrPath path.Path) {
+	message := fmt.Sprintf("%s cannot be updated", attrPath)
+	diags.AddError(message, message)
+}

internal/service/flexcluster/resource_schema.go

@@ -21,14 +21,14 @@ func ResourceSchema(ctx context.Context) schema.Schema {
 			"project_id": schema.StringAttribute{
 				Required: true,
 				PlanModifiers: []planmodifier.String{
-					customplanmodifier.NonUpdatableStringAttributePlanModifier(),
+					customplanmodifier.CreateOnlyAttributePlanModifier(),
 				},
 				MarkdownDescription: "Unique 24-hexadecimal character string that identifies the project.",
 			},
 			"name": schema.StringAttribute{
 				Required: true,
 				PlanModifiers: []planmodifier.String{
-					customplanmodifier.NonUpdatableStringAttributePlanModifier(),
+					customplanmodifier.CreateOnlyAttributePlanModifier(),
 				},
 				MarkdownDescription: "Human-readable label that identifies the instance.",
 			},
@@ -37,7 +37,7 @@ func ResourceSchema(ctx context.Context) schema.Schema {
 					"backing_provider_name": schema.StringAttribute{
 						Required: true,
 						PlanModifiers: []planmodifier.String{
-							customplanmodifier.NonUpdatableStringAttributePlanModifier(),
+							customplanmodifier.CreateOnlyAttributePlanModifier(),
 						},
 						MarkdownDescription: "Cloud service provider on which MongoDB Cloud provisioned the flex cluster.",
 					},
@@ -58,7 +58,7 @@ func ResourceSchema(ctx context.Context) schema.Schema {
 					"region_name": schema.StringAttribute{
 						Required: true,
 						PlanModifiers: []planmodifier.String{
-							customplanmodifier.NonUpdatableStringAttributePlanModifier(),
+							customplanmodifier.CreateOnlyAttributePlanModifier(),
 						},
 						MarkdownDescription: "Human-readable label that identifies the geographic location of your MongoDB flex cluster. The region you choose can affect network latency for clients accessing your databases. For a complete list of region names, see [AWS](https://docs.atlas.mongodb.com/reference/amazon-aws/#std-label-amazon-aws), [GCP](https://docs.atlas.mongodb.com/reference/google-gcp/), and [Azure](https://docs.atlas.mongodb.com/reference/microsoft-azure/).",
 					},

internal/service/project/resource_project_schema.go

@@ -15,6 +15,7 @@ import (
 	"github.com/hashicorp/terraform-plugin-framework/resource/schema/stringplanmodifier"
 	"github.com/hashicorp/terraform-plugin-framework/types"
 	"github.com/mongodb/terraform-provider-mongodbatlas/internal/common/constant"
+	"github.com/mongodb/terraform-provider-mongodbatlas/internal/common/customplanmodifier"
 	"go.mongodb.org/atlas-sdk/v20241113005/admin"
 )
 
@@ -54,9 +55,10 @@ func ResourceSchema(ctx context.Context) schema.Schema {
 			"with_default_alerts_settings": schema.BoolAttribute{
 				// Default values also must be Computed otherwise Terraform throws error:
 				// Schema Using Attribute Default For Non-Computed Attribute
-				Optional: true,
-				Computed: true,
-				Default:  booldefault.StaticBool(true),
+				Optional:      true,
+				Computed:      true,
+				Default:       booldefault.StaticBool(true),
+				PlanModifiers: []planmodifier.Bool{customplanmodifier.CreateOnlyAttributePlanModifier()},
 			},
 			"is_collect_database_specifics_statistics_enabled": schema.BoolAttribute{
 				Computed: true,