diff --git a/app/models/assignment.rb b/app/models/assignment.rb
index 3bd9644ad81..793ac7e223f 100755
--- a/app/models/assignment.rb
+++ b/app/models/assignment.rb
@@ -483,7 +483,7 @@ def self.export_headers(parent_id)
     @assignment = Assignment.find(parent_id)
     fields = []
     fields << 'Assignment Name: ' + @assignment.name.to_s
-    fields << 'Assignment Instructor: ' + User.find(@assignment.instructor_id).name.to_s
+    fields << 'Assignment Instructor: ' + User.find(@assignment.instructor_id).username.to_s
     fields
   end
 
diff --git a/app/models/team.rb b/app/models/team.rb
index 8e8edc9cf5e..6305b3c83a1 100755
--- a/app/models/team.rb
+++ b/app/models/team.rb
@@ -261,7 +261,7 @@ def self.export(csv, parent_id, options, teamtype)
       if options[:team_name] == 'false'
         team_members = TeamsUser.where(team_id: team.id)
         team_members.each do |user|
-          output.push(user.user)
+          output.push(user.username)
         end
       end
       csv << output
diff --git a/app/models/user.rb b/app/models/user.rb
index 9203eeeeaf4..e38248a9cb5 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -20,7 +20,7 @@ class User < ApplicationRecord
   belongs_to :parent, class_name: 'User'
   belongs_to :role
   validates :username, presence: true
-  validates :username, uniqueness: true
+  validates_uniqueness_of :username, :case_sensitive => true
   validates :username, format: { without: /\s/ }
 
   validates :email, presence: { message: "can't be blank" }
diff --git a/spec/models/review_response_map_spec.rb b/spec/models/review_response_map_spec.rb
index 32dc3e0f6eb..da88716371d 100755
--- a/spec/models/review_response_map_spec.rb
+++ b/spec/models/review_response_map_spec.rb
@@ -139,17 +139,17 @@
     allow(User).to receive(:find_by).and_return(nil)
     expect { ReviewResponseMap.import(row_hash, session, 1) }.to raise_error(ArgumentError, 'Cannot find reviewee user.')
     # when reviewee user exists but reviewee user is not a participant in this assignment
-    allow(User).to receive(:find_by).with(name: 'name').and_return(student)
+    allow(User).to receive(:find_by).with(username: 'name').and_return(student)
     allow(AssignmentParticipant).to receive(:find_by).with(user_id: 1, parent_id: 1).and_return(nil)
     expect { ReviewResponseMap.import(row_hash, session, 1) }.to raise_error(ArgumentError, 'Reviewee user is not a participant in this assignment.')
     # when reviewee user exists and reviewee user is a participant in this assignment
     allow(AssignmentParticipant).to receive(:find_by).with(user_id: 1, parent_id: 1).and_return(participant)
     allow(AssignmentTeam).to receive(:team).with(participant).and_return(team)
     ## when reviewer user doesn't exist
-    allow(User).to receive(:find_by).with(name: 'name1').and_return(nil)
+    allow(User).to receive(:find_by).with(username: 'name1').and_return(nil)
     expect { ReviewResponseMap.import(row_hash, session, 1) }.to raise_error(ArgumentError, 'Cannot find reviewer user.')
     ## when reviewer user exist
-    allow(User).to receive(:find_by).with(name: 'name1').and_return(student1)
+    allow(User).to receive(:find_by).with(username: 'name1').and_return(student1)
     ### when reviewer user is not a participant in this assignment.
     allow(AssignmentParticipant).to receive(:find_by).with(user_id: 2, parent_id: 1).and_return(nil)
     expect { ReviewResponseMap.import(row_hash, session, 1) }.to raise_error(ArgumentError, 'Reviewer user is not a participant in this assignment.')
@@ -166,7 +166,7 @@
     allow(TeamsUser).to receive(:create).with(team_id: 1, user_id: 1).and_return(double('teams_users', id: 1, team_id: 1, user_id: 1))
     allow(TeamNode).to receive(:create).with(parent_id: assignment_id, node_object_id: 1).and_return(double('team_node', id: 1, parent_id: 1, node_object_id: 1))
     allow(TeamUserNode).to receive(:create).with(parent_id: 1, node_object_id: 1).and_return(double('team_user_node', id: 1, parent_id: 1, node_object_id: 1))
-    allow(User).to receive(:find_by).with(name: 'name1').and_return(student1)
+    allow(User).to receive(:find_by).with(username: 'name1').and_return(student1)
     allow(AssignmentParticipant).to receive(:find_by).with(user_id: 2, parent_id: 1).and_return(participant1)
     allow(ReviewResponseMap).to receive(:find_or_create_by)
       .with(reviewed_object_id: 1, reviewer_id: 1, reviewee_id: 1, calibrate_to: false).and_return(review_response_map)