diff --git a/app/models/contest.rb b/app/models/contest.rb
index 024ca908..adb0ca6d 100644
--- a/app/models/contest.rb
+++ b/app/models/contest.rb
@@ -7,17 +7,17 @@ class Contest < ApplicationRecord
   has_many :contest_relations, dependent: :destroy
   has_many :registrants, through: :contest_relations, source: :user
 
-  has_many :contestant_records, -> { where(checked_in: true).where.not(started_at: nil) }, class_name: ContestRelation
+  has_many :contestant_records, -> { where(checked_in: true).where.not(started_at: nil) }, class_name: "ContestRelation"
   has_many :contestants, through: :contestant_records, source: :user
 
   has_many :contest_supervisors, dependent: :destroy
   has_many :supervisors, through: :contest_supervisors, source: :user
 
-  has_many :group_associations, class_name: GroupContest, inverse_of: :contest, dependent: :destroy
+  has_many :group_associations, class_name: "GroupContest", inverse_of: :contest, dependent: :destroy
   has_many :groups, through: :group_associations
   has_many :group_members, -> { distinct }, through: :groups, source: :users
 
-  belongs_to :owner, class_name: :User
+  belongs_to :owner, class_name: "User"
 
   validates :name, presence: true
 
diff --git a/app/models/contest_relation.rb b/app/models/contest_relation.rb
index 3a6658f0..2f6c61eb 100644
--- a/app/models/contest_relation.rb
+++ b/app/models/contest_relation.rb
@@ -5,7 +5,7 @@ class ContestRelation < ApplicationRecord
   belongs_to :contest
   has_many :contest_scores, dependent: :destroy
   belongs_to :school
-  belongs_to :supervisor, class_name: :User
+  belongs_to :supervisor, class_name: "User"
 
   scope :active, -> { where("started_at <= :now AND finish_at > :now", now: DateTime.now) }
   scope :absent, -> { where(checked_in: false) }
diff --git a/app/models/evaluator.rb b/app/models/evaluator.rb
index b74e3290..0aedf2f2 100644
--- a/app/models/evaluator.rb
+++ b/app/models/evaluator.rb
@@ -2,7 +2,7 @@ class Evaluator < ApplicationRecord
   include ActiveModel::ForbiddenAttributesProtection
 
   has_many :problems
-  belongs_to :owner, class_name: :User
+  belongs_to :owner, class_name: "User"
   belongs_to :language
 
   validates :name, presence: true
diff --git a/app/models/file_attachment.rb b/app/models/file_attachment.rb
index d5f22142..ee4c8ba3 100644
--- a/app/models/file_attachment.rb
+++ b/app/models/file_attachment.rb
@@ -1,7 +1,7 @@
 class FileAttachment < ApplicationRecord
   include ActiveModel::ForbiddenAttributesProtection
 
-  belongs_to :owner, class_name: :User
+  belongs_to :owner, class_name: "User"
 
   has_many :filelinks, dependent: :destroy
 
diff --git a/app/models/group.rb b/app/models/group.rb
index c36d183b..3be15b90 100644
--- a/app/models/group.rb
+++ b/app/models/group.rb
@@ -1,23 +1,23 @@
 class Group < ApplicationRecord
   include ActiveModel::ForbiddenAttributesProtection
 
-  has_many :memberships, class_name: GroupMembership, dependent: :destroy
+  has_many :memberships, class_name: "GroupMembership", dependent: :destroy
   has_many :members, through: :memberships
-  has_many :problem_set_associations, -> { includes(:problem_set).order("COALESCE(group_problem_sets.name,problem_sets.name)").references(:group_problem_sets, :problem_sets) }, class_name: GroupProblemSet, dependent: :destroy, inverse_of: :group
+  has_many :problem_set_associations, -> { includes(:problem_set).order("COALESCE(group_problem_sets.name,problem_sets.name)").references(:group_problem_sets, :problem_sets) }, class_name: "GroupProblemSet", dependent: :destroy, inverse_of: :group
   has_many :problem_sets, through: :problem_set_associations
-  has_many :contest_associations, class_name: GroupContest, inverse_of: :group, dependent: :destroy
+  has_many :contest_associations, class_name: "GroupContest", inverse_of: :group, dependent: :destroy
   has_many :contests, through: :contest_associations
 
-  belongs_to :owner, class_name: :User
+  belongs_to :owner, class_name: "User"
 
-  has_many :join_requests, -> { where(subject_type: "User", verb: "join") }, class_name: :Request, as: :target
+  has_many :join_requests, -> { where(subject_type: "User", verb: "join") }, class_name: "Request", as: :target
   # , -> { where :subject_type => :users, :verb => :join }
   # has_many :applicants, :through => :applications, :source => :subject, :source_type => 'User'
   # def applicants
   #  self.join_requests.pending.subject
   # end
 
-  has_many :invitations, -> { where(verb: "invite", target_type: "User") }, class_name: :Request, as: :subject
+  has_many :invitations, -> { where(verb: "invite", target_type: "User") }, class_name: "Request", as: :subject
   # , -> { where :verb => :invite, :target_type => :users }
   # has_many :invitees, :through => :invitations, :source => :target, :source_type => 'User', :conditions => { :status => Request::STATUS[:pending] } # TODO: expired_at condition
 
diff --git a/app/models/group_membership.rb b/app/models/group_membership.rb
index 7ff80697..213707cb 100644
--- a/app/models/group_membership.rb
+++ b/app/models/group_membership.rb
@@ -2,5 +2,5 @@ class GroupMembership < ApplicationRecord
   include ActiveModel::ForbiddenAttributesProtection
 
   belongs_to :group
-  belongs_to :member, class_name: :User
+  belongs_to :member, class_name: "User"
 end
diff --git a/app/models/item.rb b/app/models/item.rb
index dc68a9d5..3578f0c2 100644
--- a/app/models/item.rb
+++ b/app/models/item.rb
@@ -1,10 +1,10 @@
 class Item < ApplicationRecord
   belongs_to :product
-  belongs_to :owner, class_name: Entity
+  belongs_to :owner, class_name: "Entity"
   belongs_to :organisation
-  belongs_to :sponsor, class_name: Entity
-  belongs_to :donator, class_name: Entity
-  belongs_to :holder, class_name: User
+  belongs_to :sponsor, class_name: "Entity"
+  belongs_to :donator, class_name: "Entity"
+  belongs_to :holder, class_name: "User"
 
   has_many :item_histories
 
diff --git a/app/models/item_history.rb b/app/models/item_history.rb
index 665cb9ea..9553c837 100644
--- a/app/models/item_history.rb
+++ b/app/models/item_history.rb
@@ -1,6 +1,6 @@
 class ItemHistory < ApplicationRecord
   belongs_to :item
-  belongs_to :holder, class_name: User
+  belongs_to :holder, class_name: "User"
 
   # active: is the action active? (set by user borrowing book, or staff accepting return - ie. verifiable action)
   # action: enumeration
diff --git a/app/models/language.rb b/app/models/language.rb
index 895280b1..eb8e87f6 100644
--- a/app/models/language.rb
+++ b/app/models/language.rb
@@ -1,5 +1,5 @@
 class Language < ApplicationRecord
-  belongs_to :group, class_name: LanguageGroup
+  belongs_to :group, class_name: "LanguageGroup"
 
   def source_filename
     self[:source_filename] || "program"
diff --git a/app/models/language_group.rb b/app/models/language_group.rb
index e4b3f2dd..9cd6c168 100644
--- a/app/models/language_group.rb
+++ b/app/models/language_group.rb
@@ -1,4 +1,4 @@
 class LanguageGroup < ApplicationRecord
   has_many :languages, foreign_key: :group_id
-  belongs_to :current_language, class_name: Language
+  belongs_to :current_language, class_name: "Language"
 end
diff --git a/app/models/problem.rb b/app/models/problem.rb
index 65d2af5f..763b63db 100644
--- a/app/models/problem.rb
+++ b/app/models/problem.rb
@@ -1,19 +1,19 @@
 class Problem < ApplicationRecord
   include ActiveModel::ForbiddenAttributesProtection
 
-  has_many :problem_set_associations, class_name: ProblemSetProblem, inverse_of: :problem, dependent: :destroy
+  has_many :problem_set_associations, class_name: "ProblemSetProblem", inverse_of: :problem, dependent: :destroy
   has_many :problem_sets, through: :problem_set_associations
 
   has_many :test_sets, -> { rank(:problem_order) }, inverse_of: :problem, dependent: :destroy
-  has_many :prerequisite_sets, -> { where(prerequisite: true).rank(:problem_order) }, class_name: TestSet
+  has_many :prerequisite_sets, -> { where(prerequisite: true).rank(:problem_order) }, class_name: "TestSet"
   has_many :test_cases, -> { rank(:problem_order) }, inverse_of: :problem, dependent: :destroy
-  has_many :sample_cases, -> { where(sample: true).rank(:problem_order) }, class_name: TestCase
+  has_many :sample_cases, -> { where(sample: true).rank(:problem_order) }, class_name: "TestCase"
   has_many :submissions, dependent: :destroy
-  has_many :test_submissions, -> { where.not(classification: [Submission::CLASSIFICATION[:ranked], Submission::CLASSIFICATION[:unranked]]).order(:evaluation) }, class_name: Submission
+  has_many :test_submissions, -> { where.not(classification: [Submission::CLASSIFICATION[:ranked], Submission::CLASSIFICATION[:unranked]]).order(:evaluation) }, class_name: "Submission"
 
   has_many :user_problem_relations, dependent: :destroy
 
-  belongs_to :owner, class_name: :User
+  belongs_to :owner, class_name: "User"
   belongs_to :evaluator
 
   has_many :contests, through: :problem_sets
diff --git a/app/models/problem_set.rb b/app/models/problem_set.rb
index ddb53a75..82528741 100644
--- a/app/models/problem_set.rb
+++ b/app/models/problem_set.rb
@@ -1,16 +1,16 @@
 class ProblemSet < ApplicationRecord
   include ActiveModel::ForbiddenAttributesProtection
 
-  has_many :problem_associations, -> { rank(:problem_set_order) }, class_name: ProblemSetProblem, inverse_of: :problem_set, dependent: :destroy
+  has_many :problem_associations, -> { rank(:problem_set_order) }, class_name: "ProblemSetProblem", inverse_of: :problem_set, dependent: :destroy
   has_many :problems, through: :problem_associations
-  has_many :group_associations, class_name: GroupProblemSet, inverse_of: :problem_set, dependent: :destroy
+  has_many :group_associations, class_name: "GroupProblemSet", inverse_of: :problem_set, dependent: :destroy
   has_many :groups, through: :group_associations
   has_many :group_memberships, -> { distinct }, through: :groups, source: :memberships
 
   has_many :contests
   has_many :contest_relations, through: :contests, source: :contest_relations
 
-  belongs_to :owner, class_name: :User
+  belongs_to :owner, class_name: "User"
 
   accepts_nested_attributes_for :problem_associations
 
diff --git a/app/models/request.rb b/app/models/request.rb
index 8efac662..0e646046 100644
--- a/app/models/request.rb
+++ b/app/models/request.rb
@@ -1,11 +1,11 @@
 class Request < ApplicationRecord
   include ActiveModel::ForbiddenAttributesProtection
 
-  belongs_to :requester, class_name: :User # entity that initiated request
+  belongs_to :requester, class_name: "User" # entity that initiated request
   belongs_to :subject, polymorphic: true # subject controlled by requester
   # verb # action applying subject to target
   belongs_to :target, polymorphic: true # target controlled by requestee
-  belongs_to :requestee, class_name: :User # entity with primary responsibility/right to accept this request
+  belongs_to :requestee, class_name: "User" # entity with primary responsibility/right to accept this request
 
   STATUS = Enumeration.new 0 => :pending, 1 => :accepted, 2 => :rejected, 3 => :cancelled
 
diff --git a/app/models/school.rb b/app/models/school.rb
index c6ac8e43..07080152 100644
--- a/app/models/school.rb
+++ b/app/models/school.rb
@@ -1,6 +1,6 @@
 class School < ApplicationRecord
   has_many :users, dependent: :nullify
 
-  has_many :synonyms, class_name: School, as: :synonym
-  belongs_to :synonym, class_name: School
+  has_many :synonyms, class_name: "School", as: :synonym
+  belongs_to :synonym, class_name: "School"
 end
diff --git a/app/models/user.rb b/app/models/user.rb
index 759b8f0c..66bdb82b 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -29,18 +29,18 @@ class User < ApplicationRecord
   # NOTE: difference between groups and roles
   # Groups are used to assign local permissions, eg. access to individual problems/problem sets
   # Roles are used to assign global permissions, eg. access to problems on the whole site
-  has_many :memberships, class_name: :GroupMembership, foreign_key: :member_id, dependent: :destroy
+  has_many :memberships, class_name: "GroupMembership", foreign_key: :member_id, dependent: :destroy
   has_many :user_problem_relations, dependent: :destroy
   has_many :groups, through: :memberships
   has_and_belongs_to_many :roles
 
   # has_many :group_invitations, :class_name => :Request, :as => :target, :conditions => { :verb => 'invite', :subject_type => 'Group' }
-  has_many :requests, -> { where("requestee_id = target_id") }, class_name: Request, as: :target
+  has_many :requests, -> { where("requestee_id = target_id") }, class_name: "Request", as: :target
   has_one :entity, as: :entity
 
   belongs_to :school, counter_cache: true
 
-  has_many :contest_supervising, class_name: :ContestSupervisor, dependent: :destroy
+  has_many :contest_supervising, class_name: "ContestSupervisor", dependent: :destroy
 
   # Scopes
 
diff --git a/app/models/user_problem_relation.rb b/app/models/user_problem_relation.rb
index 9b343ede..ba9cf9b5 100644
--- a/app/models/user_problem_relation.rb
+++ b/app/models/user_problem_relation.rb
@@ -5,7 +5,7 @@ class UserProblemRelation < ApplicationRecord
   belongs_to :user
   # caches the score of the best ranked submission and best submission a user scored for a problem
   belongs_to :submission
-  belongs_to :ranked_submission, class_name: Submission
+  belongs_to :ranked_submission, class_name: "Submission"
 
   validates_uniqueness_of :user_id, scope: :problem_id