changing the parent_id will move group and not recreate it

this will fix #574

go.mod

@@ -2,6 +2,9 @@ module github.com/gitlabhq/terraform-provider-gitlab
 
 go 1.16
 
+// we are waiting for a new release
+replace github.com/xanzy/go-gitlab => ../go-gitlab
+
 require (
 	github.com/hashicorp/go-cty v1.4.1-0.20200414143053-d3edf31b6320
 	github.com/hashicorp/go-retryablehttp v0.7.1

internal/provider/resource_gitlab_group.go

@@ -141,7 +141,6 @@ var _ = registerResource("gitlab_group", func() *schema.Resource {
 				Description: "Id of the parent group (creates a nested group).",
 				Type:        schema.TypeInt,
 				Optional:    true,
-				ForceNew:    true,
 				Default:     0,
 			},
 			"runners_token": {
@@ -369,9 +368,40 @@ func resourceGitlabGroupUpdate(ctx context.Context, d *schema.ResourceData, meta
 		return diag.FromErr(err)
 	}
 
+	if d.HasChange("parent_id") {
+		diagnostic := transferSubGroup(d, client)
+		if diagnostic.HasError() {
+			return diagnostic
+		}
+	}
+
 	return resourceGitlabGroupRead(ctx, d, meta)
 }
 
+func transferSubGroup(d *schema.ResourceData, meta interface{}) diag.Diagnostics {
+	client := meta.(*gitlab.Client)
+	o, n := d.GetChange("parent_id")
+	parentId, ok := n.(int)
+	if !ok {
+		return diag.Errorf("error converting parent_id %v into an int", n)
+	}
+
+	opt := &gitlab.TransferSubGroupOptions{}
+	if parentId != 0 {
+		log.Printf("[DEBUG] transfer gitlab group %s from %v to new parent group %v", d.Id(), o, n)
+		opt.GroupID = gitlab.Int(parentId)
+	} else {
+		log.Printf("[DEBUG] turn gitlab group %s from %v to a new top-level group", d.Id(), o)
+	}
+
+	_, _, err := client.Groups.TransferSubGroup(d.Id(), opt)
+	if err != nil {
+		return diag.Errorf("error transfering group %s to new parent group %v: %s", d.Id(), parentId, err)
+	}
+
+	return nil
+}
+
 func resourceGitlabGroupDelete(ctx context.Context, d *schema.ResourceData, meta interface{}) diag.Diagnostics {
 	client := meta.(*gitlab.Client)
 	log.Printf("[DEBUG] Delete gitlab group %s", d.Id())

internal/provider/resource_gitlab_group_test.go

@@ -157,6 +157,7 @@ func TestAccGitlabGroup_nested(t *testing.T) {
 						TwoFactorGracePeriod:    48,           // default value
 						DefaultBranchProtection: 2,            // default value
 						Parent:                  &group,
+						AutoDevopsEnabled:       true,
 					}),
 				),
 			},
@@ -177,6 +178,7 @@ func TestAccGitlabGroup_nested(t *testing.T) {
 						TwoFactorGracePeriod:    48,           // default value
 						DefaultBranchProtection: 2,            // default value
 						Parent:                  &group2,
+						AutoDevopsEnabled:       true,
 					}),
 				),
 			},
@@ -504,6 +506,12 @@ resource "gitlab_group" "nested_foo" {
   # So that acceptance tests can be run in a gitlab organization
   # with no billing
   visibility_level = "public"
+
+  # We set this to non-default value to later check if the group was recreated or not
+  auto_devops_enabled = true
+  lifecycle {
+    ignore_changes = [auto_devops_enabled]
+  }
 }
   `, rInt, rInt, rInt, rInt, rInt, rInt)
 }
@@ -569,6 +577,11 @@ resource "gitlab_group" "nested_foo" {
   # So that acceptance tests can be run in a gitlab organization
   # with no billing
   visibility_level = "public"
+
+  # We need to read this value to check if the group was recreated or not
+  lifecycle {
+    ignore_changes = [auto_devops_enabled]
+  }
 }
   `, rInt, rInt, rInt, rInt, rInt, rInt)
 }