diff --git a/app/controllers/application_controller.rb b/app/controllers/application_controller.rb
index b280d19..2cefcba 100644
--- a/app/controllers/application_controller.rb
+++ b/app/controllers/application_controller.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class ApplicationController < ActionController::Base
   # Prevent CSRF attacks by raising an exception.
   # For APIs, you may want to use :null_session instead.
@@ -12,11 +14,11 @@ class ApplicationController < ActionController::Base
   end
 
   def glossary
-    render action: "glossary"
+    render action: 'glossary'
   end
 
   def impressum
-    render action: "impressum"
+    render action: 'impressum'
   end
 
   helper_method :current_user, :signed_in?
diff --git a/app/controllers/geo_controller.rb b/app/controllers/geo_controller.rb
index 6b6955a..50f756e 100644
--- a/app/controllers/geo_controller.rb
+++ b/app/controllers/geo_controller.rb
@@ -1,6 +1,7 @@
-class GeoController < ApplicationController
+# frozen_string_literal: true
 
+class GeoController < ApplicationController
   def index
-    render action: "index"
+    render action: 'index'
   end
 end
diff --git a/app/controllers/import_controller.rb b/app/controllers/import_controller.rb
index 4a69f67..ecf2fe6 100644
--- a/app/controllers/import_controller.rb
+++ b/app/controllers/import_controller.rb
@@ -1,11 +1,13 @@
+# frozen_string_literal: true
+
 class ImportController < ApplicationController
-  skip_before_action :verify_authenticity_token, :only => [:new_papers_callback]
+  skip_before_action :verify_authenticity_token, only: [:new_papers_callback]
 
   def new_papers_callback
     require 'open-uri'
-    api_key = Rails.application.config_for(:morph)["key"]
+    api_key = Rails.application.config_for(:morph)['key']
     uri = URI.parse("https://api.morph.io/jrlover/city_council_leipzig_recent_papers/data.json?key=#{api_key}&query=select%20*%20from%20%27data%27")
     Paper.import_from_json(uri.read)
-    render :nothing => true
+    render nothing: true
   end
-end
\ No newline at end of file
+end
diff --git a/app/controllers/search_controller.rb b/app/controllers/search_controller.rb
index 775022e..778c79f 100644
--- a/app/controllers/search_controller.rb
+++ b/app/controllers/search_controller.rb
@@ -1,14 +1,15 @@
-SearchFacet = Struct.new("SearchFacet", :term, :count) do
+# frozen_string_literal: true
+
+SearchFacet = Struct.new('SearchFacet', :term, :count) do
   def term_with_count
     "#{term} (#{count})"
   end
 end
 
-
 class SearchController < ApplicationController
   def index
     @search_definition = PaperSearch.new(search_params)
-    @search_definition.sort_by ||= "date"
+    @search_definition.sort_by ||= 'date'
 
     execute_search
   end
@@ -16,7 +17,7 @@ class SearchController < ApplicationController
   def show
     @search_definition = PaperSearch.find params[:id]
     execute_search
-    render action: "index"
+    render action: 'index'
   end
 
   private
@@ -24,35 +25,35 @@ class SearchController < ApplicationController
   def execute_search
     @response = Paper.search(@search_definition.to_definition)
     @papers = @response.page(params[:page]).results
-    @sub = Hash.new
+    @sub = {}
     @papers.each do |paper|
-      unless paper.reference.nil? && paper.reference.contains("-")
-        segments = paper.reference.split("-")
-        id = ((paper.reference.start_with?("VI-") || paper.reference.start_with?("VII-")) && segments.count > 2 ?
-          segments[2] : segments[1])
-        escaped_chars = Regexp.escape('\\+-*:()[]{}&!?^|\/')
-        sanitized_id = id.gsub(/([#{escaped_chars}])/, '\\\\\1')
-        ['AND', 'OR', 'NOT'].each do |reserved|
-          escaped_reserved = reserved.split('').map { |c| "\\#{c}" }.join('')
-          sanitized_id = sanitized_id.gsub('/\s*\b(#{reserved.upcase})\b\s*/',
-            " #{escaped_reserved} ")
-        end
-        @sub_search_definition = Elasticsearch::DSL::Search.search do
-          query do
-            query_string do
-          	  query "*" + sanitized_id + "*"
-              fields ["reference"]
-            end
-          end
+      next if paper.reference.nil? && paper.reference.contains('-')
 
-          sort do
-            by :published_at, order: 'desc'
-            by :reference, order: 'desc'
+      segments = paper.reference.split('-')
+      id = ((paper.reference.start_with?('VI-') || paper.reference.start_with?('VII-')) && segments.count > 2 ?
+        segments[2] : segments[1])
+      escaped_chars = Regexp.escape('\\+-*:()[]{}&!?^|\/')
+      sanitized_id = id.gsub(/([#{escaped_chars}])/, '\\\\\1')
+      %w[AND OR NOT].each do |reserved|
+        escaped_reserved = reserved.split('').map { |c| "\\#{c}" }.join('')
+        sanitized_id = sanitized_id.gsub('/\s*\b(#{reserved.upcase})\b\s*/',
+                                         " #{escaped_reserved} ")
+      end
+      @sub_search_definition = Elasticsearch::DSL::Search.search do
+        query do
+          query_string do
+            query '*' + sanitized_id + '*'
+            fields ['reference']
           end
         end
-        @sub_papers = Paper.search(@sub_search_definition)
-        @sub[paper.reference] = @sub_papers
+
+        sort do
+          by :published_at, order: 'desc'
+          by :reference, order: 'desc'
+        end
       end
+      @sub_papers = Paper.search(@sub_search_definition)
+      @sub[paper.reference] = @sub_papers
     end
     @paper_type_facets = extract_facets('paper_types')
     @originator_facets = extract_facets('originators')
@@ -63,9 +64,8 @@ class SearchController < ApplicationController
   end
 
   def extract_facets(name)
-    @response.
-      response['aggregations'][name.to_s][name.to_s]['buckets'].
-      map {|m| SearchFacet.new(m['key'], m['doc_count'])}
+    @response
+      .response['aggregations'][name.to_s][name.to_s]['buckets']
+      .map { |m| SearchFacet.new(m['key'], m['doc_count']) }
   end
-
 end
diff --git a/app/helpers/search_helper.rb b/app/helpers/search_helper.rb
index 1eee228..79f1e60 100644
--- a/app/helpers/search_helper.rb
+++ b/app/helpers/search_helper.rb
@@ -1,26 +1,27 @@
+# frozen_string_literal: true
+
 module SearchHelper
   def facet_list(facets)
     return unless facets.present?
+
     content_tag(:ul) do
       facets.each do |facet|
         concat content_tag(:li,
                            "#{facet.term} (#{facet.count})",
-                           class: "facet"
-                          )
+                           class: 'facet')
       end
     end
   end
 
   def filter_select(builder, name, desc, facets, selected)
     capture do
-      concat(builder.label name, desc)
+      concat(builder.label(name, desc))
       concat(
-        builder.select name,
-          options_from_collection_for_select(facets, :term, :term_with_count, selected),
-          { include_blank: true },
-          { onchange: "this.form.submit();" }
+        builder.select(name,
+                       options_from_collection_for_select(facets, :term, :term_with_count, selected),
+                       { include_blank: true },
+                       { onchange: 'this.form.submit();' })
       )
     end
   end
-
 end
diff --git a/app/models/importer.rb b/app/models/importer.rb
index f58649b..96fde1f 100644
--- a/app/models/importer.rb
+++ b/app/models/importer.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class Importer < ActiveRecord::Base
   validates :url, presence: true, uniqueness: true
 end
diff --git a/app/models/paper.rb b/app/models/paper.rb
index 80c2fa8..94fdf86 100644
--- a/app/models/paper.rb
+++ b/app/models/paper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'elasticsearch/model'
 require 'json'
 require 'parseable_date_validator'
@@ -9,7 +11,7 @@ class Paper < ActiveRecord::Base
   validates :name,         presence: true, length: { maximum: 1000 }
   validates :url,          presence: true,
                            length: { maximum: 1000 },
-                           uniqueness: true, # TODO use unique index instead
+                           uniqueness: true, # TODO: use unique index instead
                            url: true
   validates :reference,    presence: true, length: { maximum: 100 }
   validates :body,         presence: true, length: { maximum: 100 }
@@ -19,45 +21,46 @@ class Paper < ActiveRecord::Base
   validates :published_at, presence: true, parseable_date: true
   validates :resolution,   length: { maximum: 30_000 }
 
-  index_name ['srm', Rails.env, self.base_class.to_s.pluralize.underscore].join('_')
+  index_name ['srm', Rails.env, base_class.to_s.pluralize.underscore].join('_')
 
   settings index: {
     number_of_shards: 1,
     analysis: {
       filter: {
         german_stop: {
-          type: "stop",
-          stopwords: "_german_"
+          type: 'stop',
+          stopwords: '_german_'
         },
         german_stemmer: {
-          type: "stemmer",
-          language: "light_german"
+          type: 'stemmer',
+          language: 'light_german'
         },
         german_decompounder: {
-          type: "hyphenation_decompounder",
-          word_list_path: "analysis/dictionary-de.txt",
-          hyphenation_patterns_path: "analysis/de_DR.xml",
+          type: 'hyphenation_decompounder',
+          word_list_path: 'analysis/dictionary-de.txt',
+          hyphenation_patterns_path: 'analysis/de_DR.xml',
           only_longest_match: true,
           min_subword_size: 4
-        },
+        }
       },
       analyzer: {
         german: {
-          tokenizer: "standard",
-          filter: [
-            "lowercase",
-            "german_stop",
-              "german_decompounder",
-            "german_normalization",
-            "german_stemmer"
+          tokenizer: 'standard',
+          filter: %w[
+            lowercase
+            german_stop
+            german_decompounder
+            german_normalization
+            german_stemmer
           ]
         }
       }
     }
-    } do mappings dynamic: false do
-      indexes :name, type: :text, analyzer: "german"
-      indexes :content, type: :text, analyzer: "german"
-      indexes :resolution, type: :text, analyzer: "german"
+  } do
+    mappings dynamic: false do
+      indexes :name, type: :text, analyzer: 'german'
+      indexes :content, type: :text, analyzer: 'german'
+      indexes :resolution, type: :text, analyzer: 'german'
       indexes :reference, type: :keyword, index: true
       indexes :paper_type, type: :keyword, index: true
       indexes :published_at, type: :date, index: true
@@ -66,10 +69,10 @@ class Paper < ActiveRecord::Base
   end
 
   def split_originator
-    originator.split(/\d\.\s/).reject {|s| s.blank?} || originator
+    originator.split(/\d\.\s/).reject(&:blank?) || originator
   end
 
-  def as_indexed_json(options={})
+  def as_indexed_json(_options = {})
     as_json.merge(originator: split_originator)
   end
 
@@ -86,7 +89,7 @@ class Paper < ActiveRecord::Base
           paper_type: record['paper_type'],
           published_at: record['published_at'],
           reference: record['reference'],
-          url: record['url'],
+          url: record['url']
         }
         record = find_or_initialize_by(url: attributes[:url])
         record.update_attributes(attributes)
@@ -104,8 +107,7 @@ class Paper < ActiveRecord::Base
 
     def reset_index!
       __elasticsearch__.create_index! force: true
-      all.each {|p| p.__elasticsearch__.index_document }
+      all.each { |p| p.__elasticsearch__.index_document }
     end
-
   end
 end
diff --git a/app/models/paper_search.rb b/app/models/paper_search.rb
index 96a40d1..e9672ce 100644
--- a/app/models/paper_search.rb
+++ b/app/models/paper_search.rb
@@ -1,39 +1,43 @@
-class PaperSearch < ActiveRecord::Base
+# frozen_string_literal: true
 
+class PaperSearch < ActiveRecord::Base
   def to_definition
-    options = {paper_type: paper_type, originator: originator, sort_by: sort_by}
+    options = { paper_type: paper_type, originator: originator, sort_by: sort_by }
     PaperSearch.definition(query, options)
   end
 
-  def self.definition(q, options={})
+  def self.definition(q, options = {})
     Elasticsearch::DSL::Search.search do
-
       sort do
-        if options[:sort_by] == 'score'
-          by '_score'
-        end
+        by '_score' if options[:sort_by] == 'score'
         by :published_at, order: 'desc'
       end
 
       query do
         # search query
-        unless q.blank?
+        if q.blank?
+          match_all
+        else
           multi_match do
             query q
-            fields ["name", "content"]
+            fields %w[name content]
           end
-        else
-          match_all
         end
       end
 
       # apply filter after aggregations
       post_filter do
         bool do
-          must { term paper_type: options[:paper_type] } if options[:paper_type].present?
-          must { term originator: options[:originator] } if options[:originator].present?
+          if options[:paper_type].present?
+            must { term paper_type: options[:paper_type] }
+          end
+          if options[:originator].present?
+            must { term originator: options[:originator] }
+          end
           # catchall when no filters set
-          must { match_all } unless (options[:paper_type].present? || options[:originator].present?)
+          unless options[:paper_type].present? || options[:originator].present?
+            must { match_all }
+          end
         end
       end
 
@@ -41,7 +45,9 @@ class PaperSearch < ActiveRecord::Base
         # filter by originator
         f = Elasticsearch::DSL::Search::Filters::Bool.new
         f.must { match_all }
-        f.must { term originator: options[:originator] } if options[:originator].present?
+        if options[:originator].present?
+          f.must { term originator: options[:originator] }
+        end
         filter f.to_hash do
           aggregation :paper_types do
             terms do
@@ -55,7 +61,9 @@ class PaperSearch < ActiveRecord::Base
         # filter by paper_type
         f = Elasticsearch::DSL::Search::Filters::Bool.new
         f.must { match_all }
-        f.must { term paper_type: options[:paper_type] } if options[:paper_type].present?
+        if options[:paper_type].present?
+          f.must { term paper_type: options[:paper_type] }
+        end
         filter f.to_hash do
           aggregation :originators do
             terms do
diff --git a/app/models/user.rb b/app/models/user.rb
index 7935ea0..17de7a5 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class User < ActiveRecord::Base
   def self.find_or_create_from_auth_hash(hash)
     User.find_or_create_by(email: hash['info']['email'])
diff --git a/app/views/search/index.rss.builder b/app/views/search/index.rss.builder
index 6c89738..e2571f7 100644
--- a/app/views/search/index.rss.builder
+++ b/app/views/search/index.rss.builder
@@ -1,35 +1,37 @@
+# frozen_string_literal: true
+
 require 'date'
 
-xml.instruct! :xml, :version => "1.0"
-xml.rss :version => "2.0", "xmlns:dc" => "http://purl.org/dc/elements/1.1/" do
+xml.instruct! :xml, version: '1.0'
+xml.rss :version => '2.0', 'xmlns:dc' => 'http://purl.org/dc/elements/1.1/' do
   xml.channel do
-    xml.title "Search results"
-    xml.description "Papers matching search criteria"
+    xml.title 'Search results'
+    xml.description 'Papers matching search criteria'
 
     @papers.each do |doc|
       xml.item do
         xml.title doc.name
-        if !doc.content.blank?
-        	xml.description do 
-        		xml.cdata! truncate(doc.content.sub("------------------------------- ", ""), length: 768)
-        	end
+        unless doc.content.blank?
+          xml.description do
+            xml.cdata! truncate(doc.content.sub('------------------------------- ', ''), length: 768)
+          end
         end
-	    if !doc.published_at.blank?
-    	    xml.pubDate DateTime.parse(doc.published_at).utc.strftime("%a, %d %b %Y %H:%M:%S %z")
+        unless doc.published_at.blank?
+          xml.pubDate DateTime.parse(doc.published_at).utc.strftime('%a, %d %b %Y %H:%M:%S %z')
+          end
+        doc.originator.each do |originator|
+          xml.dc :creator do
+            xml.cdata! originator
+          end
         end
-        doc.originator.each do |originator| 
-        	xml.dc :creator do
-        		xml.cdata! originator
-        	end
-        end
-        if !doc.paper_type.blank?
-	        xml.category do 
-	        	xml.cdata! doc.paper_type
-	        end
+        unless doc.paper_type.blank?
+          xml.category do
+            xml.cdata! doc.paper_type
+          end
         end
         xml.link doc.url
         xml.guid doc.url
       end
     end
   end
-end
\ No newline at end of file
+end