diff --git a/server/controllers/LearnerController.js b/server/controllers/LearnerController.js
index 3a7e960..c3ed4c2 100644
--- a/server/controllers/LearnerController.js
+++ b/server/controllers/LearnerController.js
@@ -24,7 +24,7 @@ module.exports = {
       await learner.save();
       if (learner.mentor) throw new Error('Learner already has a mentor');
       let mentor = (await Mentor.aggregate()
-        .match({ isAvailable: true, isValidated: true })
+        .match({ isAvailable: true, isValidated: true, _id: { $nin: learner.noAssociations } })
         .group({
           _id: '$_id',
           size: { $max: '$learners' },
@@ -77,10 +77,12 @@ module.exports = {
       if (!learner.mentor) throw new Error('Learner does not have a mentor');
       await Mentor.findByIdAndUpdate(learner.mentor, {
         $pull: { learners: learner._id },
+        $push: { noAssociations: learner._id },
         isAvailable: false,
       });
       const oldMentor = learner.mentor;
       const mentorMail = await Mentor.findById(learner.mentor);
+      learner.noAssociations.push(learner.mentor);
       learner.mentor = null;
       await learner.save();
       const data = mail.unassignMentor(req.user.email, req.user.name, mentorMail.name, mentorMail.gender); // eslint-disable-line max-len
@@ -101,13 +103,24 @@ module.exports = {
       const hasLearnerCertificate = reqUser.courseCertificates.length > 0;
 
       if (!hasLearnerCertificate) throw new Error('User did not conclude Tutorial');
-
-      const user = await User.findOneAndUpdate(
-        { _id },
-        { $set: { userType: 'Mentor' } }, { new: true },
+      if (reqUser.mentor) {
+        await User.findOneAndUpdate({ _id }, {
+          $pull: { learners: reqUser._id },
+          $push: { noAssociations: reqUser._id },
+          isAvailable: false,
+        });
+        reqUser.noAssociations.push(reqUser.mentor);
+        reqUser.mentor = null;
+        reqUser.save();
+      }
+      const user = await User.findByIdAndUpdate(
+        _id,
+        {
+          $set: { userType: 'Mentor' },
+          mentor_request: false,
+        }, { new: true },
       );
       user.isValidated = true;
-      user.mentor_request = false;
       user.save();
       const data = mail.learnerPromotion(user.email, user.name);
       await transport.sendMail(data);
diff --git a/server/controllers/MentorController.js b/server/controllers/MentorController.js
index f9ceb2e..12e1c4c 100644
--- a/server/controllers/MentorController.js
+++ b/server/controllers/MentorController.js
@@ -21,13 +21,21 @@ module.exports = {
 
   async assignLearner(req, res) {
     const { user } = req;
+    if (!user.isValidated) throw new Error('Mentor not validated');
     user.isAvailable = true;
     try {
       await user.save();
       const learner = (
-        await Learner.find({ mentor_request: true, mentor: null }).sort({
-          createdAt: 'asc',
+        await Learner.find({
+          mentor_request: true,
+          mentor: null,
+          _id: {
+            $nin: user.noAssociations,
+          },
         })
+          .sort({
+            createdAt: 'asc',
+          })
       )[0];
       if (!learner) throw new Error("There's no available learners");
       learner.mentor = user._id;
@@ -66,8 +74,10 @@ module.exports = {
       const learner = await Learner.findByIdAndUpdate(learnerID, {
         mentor: null,
         mentor_request: false,
+        $push: { noAssociations: user._id },
       },
       { new: true });
+      user.noAssociations.push(learnerID);
       await user.save();
       await user.execPopulate('learners');
       const data = mail.unassignMentor(learner.email, learner.name, user.name, user.gender);
diff --git a/server/models/User.js b/server/models/User.js
index acd3ab3..43b9a1a 100644
--- a/server/models/User.js
+++ b/server/models/User.js
@@ -45,10 +45,11 @@ const UserSchema = new Schema(
       type: Boolean,
       select: false,
     },
-    noAssociations: {
-      type: [String],
+    noAssociations: [{
+      type: mongoose.Schema.Types.ObjectId,
+      ref: 'blacklistedUser',
       select: false,
-    },
+    }],
     courseCertificates: [{
       type: mongoose.Schema.Types.ObjectId,
       ref: 'courseCertificate',