User.send_message = function (id, body, callback) {

    var error = new Error("Access denied.");
    error.errorCode = 401;
    var paticipants = [];

    if (typeof body.participants === 'undefined' || body.participants === null || !Array.isArray(body.participants) || !body.participants.length) {
      return callback(error);
    } else {

      var userIds = body.participants;
      userIds.push(id);
      User.find({where: {id: {inq: userIds}}}, function (err, users) {

        if (users && users.length && err === null) {
          // found users
          users.forEach(function (user) {
            paticipants.push(user.id);
          });


          // now let do query to mongodb.

          var GroupCollection = User.getDataSource().connector.collection(User.app.models.group.modelName);
          GroupCollection.aggregate([
            {$match: {status: true}},
            {$sort: {updatedAt: 1}},
            {
              $lookup: {localField: "_id", from: "member", foreignField: "groupId", as: "members"}
            },
            {
              $group: {
                _id: "$_id",
                members: {$addToSet: "$members.userId"}
              }
            },
            {
              $project: {
                _id: 1,
                id: 1,
                title: 1,
                members: {$arrayElemAt: ["$members", 0]}
              }
            },
            {
              $match: {
                $and: [
                  {members: {$all: paticipants}},
                  {members: {$size: paticipants.length}}
                ]
              }
            },

          ], function (err, result) {

            var filterQuery = {
              include: [
                {
                  relation: "messages",
                  scope: {
                    order: ["createdAt DESC"],
                    limit: 10
                  }
                },
                {
                  relation: "members",
                }
              ]
            };


            if (err === null && result && result.length && typeof result[0]._id !== 'undefined' && result[0]._id !== null) {
              // found item.



              // if have message in body we need create it

              if (typeof body.message !== 'undefined' && body.message !== null && typeof body.message.body !== 'undefined' && body.message.body !== null && body.message.body !== "") {
                User.app.models.message.create({
                  userId: id,
                  body: body.message.body,
                  messageType: body.message.messageType ? body.message.messageType : "text",
                  groupId: result[0]._id
                }, function (err, newMessage) {

                  if (err !== null) {
                    return callback(err);
                  } else {

                    User.app.models.group.findById(result[0]._id, filterQuery, function (err, group) {
                      return callback(null, group);
                    });
                  }


                });

              } else {

                User.app.models.group.findById(result[0]._id, filterQuery, function (err, group) {
                  return callback(null, group);
                });

              }


            } else {
              // we need create new group. if message from body is found
              if (typeof body.message !== 'undefined' && body.message !== null && typeof body.message.body !== 'undefined' && body.message.body !== null && body.message.body !== "") {


                User.app.models.group.create({
                  userId: id
                }, function (err, group) {

                  if (err) {
                    return callback(err);
                  }
                  if (group) {

                    var groupMemers = [];
                    users.forEach(function (p) {
                      if (p.id != id) {
                        groupMemers.push({groupId: group.id, userId: p.id, name: p.firstName ? p.firstName : ""});
                      }
                    });

                    group.members.create(groupMemers, function (err, members) {

                      group.messages.create({
                        userId: id,
                        body: body.message.body,
                        messageType: body.message.messageType ? body.message.messageType : "text",
                        groupId: group.id
                      }, function (err, message) {


                        User.app.models.group.findById(group.id, filterQuery, function (err, group) {
                          return callback(null, group);
                        });





                      });


                    });


                  } else {
                    return callback(error);
                  }

                });

              } else {

                return callback(null, result); // this is empty group. and no message post in body.
              }


            }

          });


        } else {
          return callback(error);
        }

      });


    }


  };