diff --git a/changelogs/unreleased/sh-recover-ee-schema-backport-migration-failure.yml b/changelogs/unreleased/sh-recover-ee-schema-backport-migration-failure.yml
new file mode 100644
index 0000000000000000000000000000000000000000..1695e2827eba27f3a0c9be4df03ab0cbacd53aa9
--- /dev/null
+++ b/changelogs/unreleased/sh-recover-ee-schema-backport-migration-failure.yml
@@ -0,0 +1,5 @@
+---
+title: Prevent EE backport migrations from running if CE is not migrated
+merge_request: 30002
+author:
+type: fixed
diff --git a/db/migrate/20190402150158_backport_enterprise_schema.rb b/db/migrate/20190402150158_backport_enterprise_schema.rb
index 610a8808383d32d3a2f663b8ea465d7f01bdee77..8762cc53ed71a27f21bbad565aac2fe2a8ea0673 100644
--- a/db/migrate/20190402150158_backport_enterprise_schema.rb
+++ b/db/migrate/20190402150158_backport_enterprise_schema.rb
@@ -117,6 +117,8 @@ class BackportEnterpriseSchema < ActiveRecord::Migration[5.0]
   end
 
   def up
+    check_schema!
+
     create_missing_tables
 
     update_appearances
@@ -868,6 +870,52 @@ class BackportEnterpriseSchema < ActiveRecord::Migration[5.0]
     remove_column_if_exists(:geo_nodes, :internal_url)
   end
 
+  # Some users may have upgraded to EE at some point but downgraded to
+  # CE v11.11.3.  As a result, their EE tables may not be in the right
+  # state. Here we check for these such cases and attempt to guide the
+  # user into recovering from this state by upgrading to v11.11.3 EE
+  # before installing v12.0.0 CE.
+  def check_schema!
+    # The following cases will fail later when this migration attempts
+    # to add a foreign key for non-existent columns.
+    columns_to_check = [
+      [:epics, :parent_id], # Added in GitLab 11.7
+      [:geo_event_log, :cache_invalidation_event_id], # Added in GitLab 11.4
+      [:vulnerability_feedback, :merge_request_id] # Added in GitLab 11.9
+    ].freeze
+
+    columns_to_check.each do |table, column|
+      check_ee_columns!(table, column)
+    end
+  end
+
+  def check_ee_columns!(table, column)
+    return unless table_exists?(table)
+    return if column_exists?(table, column)
+
+    raise_ee_migration_error!(table, column)
+  end
+
+  def raise_ee_migration_error!(table, column)
+    message = "Your database is missing the '#{column}' column from the '#{table}' table that is present for GitLab EE."
+
+    message +=
+      if ::Gitlab.ee?
+        "\nUpgrade your GitLab instance to 11.11.3 EE first!"
+      else
+        <<~MSG
+
+          Even though it looks like you're running a CE installation, it appears
+          you may have installed GitLab EE at some point. To migrate to GitLab 12.0:
+
+          1. Install GitLab 11.11.3 EE
+          2. Install GitLab 12.0.x CE
+        MSG
+      end
+
+    raise Exception.new(message)
+  end
+
   def create_missing_tables
     create_table_if_not_exists "approval_merge_request_rule_sources", id: :bigserial do |t|
       t.bigint "approval_merge_request_rule_id", null: false
diff --git a/spec/migrations/backport_enterprise_schema_spec.rb b/spec/migrations/backport_enterprise_schema_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..8d2d9d4953a6816fd69f019f6ba6d140d615ac57
--- /dev/null
+++ b/spec/migrations/backport_enterprise_schema_spec.rb
@@ -0,0 +1,41 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+require Rails.root.join('db', 'migrate', '20190402150158_backport_enterprise_schema.rb')
+
+describe BackportEnterpriseSchema, :migration, schema: 20190329085614 do
+  include MigrationsHelpers
+
+  def drop_if_exists(table)
+    active_record_base.connection.drop_table(table) if active_record_base.connection.table_exists?(table)
+  end
+
+  describe '#up' do
+    it 'creates new EE tables' do
+      migrate!
+
+      expect(active_record_base.connection.table_exists?(:epics)).to be true
+      expect(active_record_base.connection.table_exists?(:geo_nodes)).to be true
+    end
+
+    context 'missing EE columns' do
+      before do
+        drop_if_exists(:epics)
+
+        active_record_base.connection.create_table "epics" do |t|
+          t.integer :group_id, null: false, index: true
+          t.integer :author_id, null: false, index: true
+        end
+      end
+
+      after do
+        drop_if_exists(:epics)
+      end
+
+      it 'flags an error' do
+        expect { migrate! }.to raise_error(/Your database is missing.*that is present for GitLab EE/)
+      end
+    end
+  end
+end