diff --git a/models/pool_version.rb b/models/pool_version.rb
index b4f5210..43c324d 100644
--- a/models/pool_version.rb
+++ b/models/pool_version.rb
@@ -13,11 +13,13 @@ class PoolVersion < ActiveRecord::Base
     pool: 5
   )
 
-  def self.find_previous(pool_id, updated_at)
-    PoolVersion.where(pool_id: pool_id).where("updated_at < ?", updated_at).order("id desc").first
+  def self.find_previous(pool_id, version = nil)
+    scope = where(pool_id: pool_id)
+    scope = scope.where("version < ?", version) if version
+    scope.order(version: :desc).first
   end
 
-  def self.calculate_version(pool_id, updated_at)
+  def self.calculate_version(pool_id)
     1 + where("pool_id = ?", pool_id).maximum(:version).to_i
   end
 
@@ -25,13 +27,13 @@ def self.create_from_json(json)
     created_at = json["created_at"] ? Time.parse(json["created_at"]) : nil
     updated_at = json["updated_at"] ? Time.parse(json["updated_at"]) : created_at
     post_ids = json["post_ids"]
-    previous = find_previous(json["pool_id"], updated_at)
+    previous = find_previous(json["pool_id"])
     subject = PoolVersion.new
-    subject.version = calculate_version(json["pool_id"], updated_at)
+    subject.version = calculate_version(json["pool_id"])
 
     if previous && previous.updater_id == json["updater_id"] && updated_at - previous.updated_at < 1.hour
       subject = previous
-      previous = find_previous(previous.pool_id, previous.updated_at)
+      previous = find_previous(previous.pool_id, previous.version)
     end
 
     if previous
diff --git a/models/post_version.rb b/models/post_version.rb
index d2cbf18..3b421ae 100644
--- a/models/post_version.rb
+++ b/models/post_version.rb
@@ -13,11 +13,13 @@ class PostVersion < ActiveRecord::Base
     pool: 5
   )
 
-  def self.find_previous(post_id, updated_at)
-    where("post_id = ? and updated_at < ?", post_id, updated_at).order("id desc").first
+  def self.find_previous(post_id, version = nil)
+    scope = where(post_id: post_id)
+    scope = scope.where("version < ?", version) if version
+    scope.order(version: :desc).first
   end
 
-  def self.calculate_version(post_id, updated_at)
+  def self.calculate_version(post_id)
     1 + where("post_id = ?", post_id).maximum(:version).to_i
   end
 
@@ -25,13 +27,13 @@ def self.create_from_json(json)
     created_at = json["created_at"] ? Time.parse(json["created_at"]) : nil
     updated_at = json["updated_at"] ? Time.parse(json["updated_at"]) : created_at
     tags = json["tags"].scan(/\S+/)
-    previous = find_previous(json["post_id"], updated_at)
+    previous = find_previous(json["post_id"])
     subject = PostVersion.new
-    subject.version = calculate_version(json["post_id"], updated_at)
+    subject.version = calculate_version(json["post_id"])
 
     if previous && previous.updater_id == json["updater_id"] && updated_at - previous.updated_at < 1.hour
       subject = previous
-      previous = find_previous(previous.post_id, previous.updated_at)
+      previous = find_previous(previous.post_id, previous.version)
     end
 
     if previous