Metatag upgrade path

gistfile1.diff

diff --git a/metatag.install b/metatag.install
index 777b265..1d83005 100644
--- a/metatag.install
+++ b/metatag.install
@@ -259,10 +259,12 @@ function metatag_update_7001() {
     'description' => 'The primary identifier for a metatag configuration set.',
   );
   $keys = array('primary key' => array($field_name));
-
-  // Before making any changes, drop the existing primary key. Unforunately
-  // there is no API way to check if a primary key exists, so if it doesn't
-  // exist the db_drop_primary_key() call will fail.
+  // Before making any changes, drop the existing primary key.
+  // Let's add a temporary unique key for cid so MySQL will let it go.
+  // Hint taken from https://drupal.org/node/2064305#comment-7753197.
+  db_add_unique_key($table_name, 'temp_key', array($field_name, 'instance'));
+  // Unforunately there is no API way to check if a primary key exists, so if
+  // it doesn't exist the db_drop_primary_key() call will fail.
   try {
     db_drop_primary_key($table_name);
   }
@@ -272,6 +274,12 @@ function metatag_update_7001() {
 
   // Rejig the field, and turn on the primary key again.
   db_change_field($table_name, $field_name, $field_name, $field_spec, $keys);
+
+  // Finally, remove the temporary unique key because it's no longer userful.
+  db_drop_unique_key($table_name, 'temp_key');
+
+  // Finally, remove the temporary unique key because it's no longer userful.
+  db_drop_unique_key($table_name, 'temp_key');
 }
 
 /**
@@ -431,6 +439,10 @@ function metatag_update_7007() {
   }
 }
 
+function metatag_skip_update_7015() {
+  variable_set('metatag_skip_update_7015', TRUE);
+}
+
 /**
  * Remove any empty records that may be hanging around from old releases.
  */
@@ -452,6 +464,9 @@ function metatag_update_7008() {
  */
 function metatag_update_7009() {
   if (module_exists('taxonomy')) {
+    // Fix the metatag table.
+    metatag_update_7015();
+    metatag_skip_update_7015();
     // Remove duplicates.
     _metatag_remove_dupes('taxonomy_term');
   }
@@ -468,6 +483,9 @@ function metatag_update_7009() {
  * Fix {metatag} records for users.
  */
 function metatag_update_7010() {
+  // Fix the metatag table.
+  metatag_update_7015();
+  metatag_skip_update_7015();
   // Remove duplicates.
   _metatag_remove_dupes('user');
 
@@ -482,6 +500,8 @@ function metatag_update_7010() {
  * Fix {metatag} records for nodes.
  */
 function metatag_update_7011(&$sandbox) {
+  metatag_update_7015();
+  metatag_skip_update_7015();
   // Only proceed if Entity_Translation is not enabled as it allows each node
   // record to have multiple languages available.
   if (module_exists('entity_translation')) {
@@ -591,6 +611,8 @@ function metatag_update_7011(&$sandbox) {
  * Remove duplicate {metatag} records for non-core entities.
  */
 function metatag_update_7012() {
+  metatag_update_7015();
+  metatag_skip_update_7015();
   if (module_exists('entity_translation')) {
     drupal_set_message(t("Entity Translation is enabled, duplicate meta tags will not be removed for custom entities, to avoid accidental dataloss."));
     return;
@@ -621,6 +643,8 @@ function metatag_update_7012() {
  * take a while, depending on how much data needs to be converted.
  */
 function metatag_update_7013(&$sandbox) {
+  metatag_update_7015();
+  metatag_skip_update_7015();
   if (module_exists('entity_translation')) {
     drupal_set_message(t("Entity Translation is enabled, meta tags will not be updated for custom entities, to avoid accidental dataloss."));
     return;
@@ -912,26 +936,31 @@ function metatag_update_7014() {
  * keys accordingly.
  */
 function metatag_update_7015() {
-  // Tell update 7016 that it isn't needed.
-  variable_set('metatag_skip_update_7017', TRUE);
+  if (!variable_get('metatag_skip_update_7015', FALSE)) {
+    // Tell update 7016 that it isn't needed.
+    variable_set('metatag_skip_update_7017', TRUE);
 
-  // Add the new field.
-  db_add_field('metatag', 'revision_id', array(
-      'type' => 'int',
-      'unsigned' => TRUE,
-      'not null' => TRUE,
-      'default' => 0,
-      'description' => 'The revision_id for the entity object this data is attached to.',
-    ));
+    // Add the new field.
+    db_add_field('metatag', 'revision_id', array(
+        'type' => 'int',
+        'unsigned' => TRUE,
+        'not null' => TRUE,
+        'default' => 0,
+        'description' => 'The revision_id for the entity object this data is attached to.',
+      ));
 
-  // Remove the existing primary key. This may take some work so it can be
-  // database agnostic, i.e. some databases will not like it.
-  db_drop_primary_key('metatag');
+    // Remove the existing primary key. This may take some work so it can be
+    // database agnostic, i.e. some databases will not like it.
+    db_drop_primary_key('metatag');
 
-  // Add the new primary key.
-  db_add_primary_key('metatag', array('entity_type', 'entity_id', 'revision_id', 'language'));
+    // Add the new primary key.
+    db_add_primary_key('metatag', array('entity_type', 'entity_id', 'revision_id', 'language'));
 
-  drupal_set_message(t('Added the {metatag}.revision_id field.'));
+    drupal_set_message(t('Added the {metatag}.revision_id field.'));
+  }
+  else {
+    variable_del('metatag_skip_update_7015');
+  }
 }
 
 /**
@@ -949,7 +978,7 @@ function metatag_update_7017() {
   if (!variable_get('metatag_skip_update_7017', FALSE)) {
     // Let's add a temporary unique key so MySQL will let it go.
     db_add_unique_key('metatag', 'temp_key', array('entity_type', 'entity_id', 'revision_id', 'language'));
-    
+
     // Now remove the PK before changing a field from serial.
     db_drop_primary_key('metatag');