Adding a unique index to migrations (#815)

* Adding an index to migrations with a unique constraint to avoid duplicate versions. Fixes #814

* Using the test db for this spec since the sample db seems to fail in CI

spec/avram/migrator/runner_spec.cr

@@ -0,0 +1,14 @@
+require "../../spec_helper"
+
+describe Avram::Migrator::Runner do
+  describe "setup_migration_tracking_table" do
+    it "includes a unique index" do
+      Avram.temp_config(database_to_migrate: TestDatabase) do
+        Avram::Migrator::Runner.setup_migration_tracking_tables
+
+        results = TestDatabase.query_all("SELECT indexname FROM pg_catalog.pg_indexes WHERE tablename = 'migrations'", as: String)
+        results.should contain("migrations_version_index")
+      end
+    end
+  end
+end

src/avram/migrator/runner.cr

@@ -103,7 +103,9 @@ class Avram::Migrator::Runner
   end
 
   def self.setup_migration_tracking_tables
-    Avram.settings.database_to_migrate.exec create_table_for_tracking_migrations
+    db = Avram.settings.database_to_migrate
+    db.exec create_table_for_tracking_migrations
+    db.exec create_unique_index_for_migrations
   end
 
   private def self.create_table_for_tracking_migrations
@@ -115,6 +117,13 @@ class Avram::Migrator::Runner
     SQL
   end
 
+  private def self.create_unique_index_for_migrations
+    <<-SQL
+    CREATE UNIQUE INDEX IF NOT EXISTS migrations_version_index
+    ON #{MIGRATIONS_TABLE_NAME} (version)
+    SQL
+  end
+
   def self.run(command : String, output : IO = STDOUT)
     error_messages = IO::Memory.new
     ENV["PGPASSWORD"] = self.db_password if self.db_password