ruby - create method adding 2 values and assigning to to an attribute in rails table -

i trying previous balance of row , add variable add params input value , assign specific column attribute. if rake db:migrate , table empty acctbal nil , bomb out. how attribute 0 first time current_user has deposited funds, such following deposit can build off of it. seen in code below, second "+" when adding 2 values gives me undefined nil error want concatenate both values, how can achieve this?

def create  #@account = account.new(account_params)  # @account.save  # respond_with(@account)    @previous_balance = account.where('user_id = ?',      current_user.id).order(:created_at).last.acctbal   @account = account.new(account_params)   @account.email = current_user.email   @account.user_id = current_user.id   @account.acctbal = account_params[:deposit] + @previous_balance   respond_to |format|      if @account.save        format.html { redirect_to accounts_url, notice: 'thank ,          enjoy.' }        format.json { render :show, status: :created, location: @account }   else     format.html { render :new }     format.json { render json: @account.errors, status:      :unprocessable_entity }         end       end    end 

my form partial:

<%= form_for(@account) |f| %>  <% if @account.errors.any? %>  <div id="error_explanation">   <h2><%= pluralize(@account.errors.count, "error") %> prohibited    account being saved:</h2>    <ul>   <% @account.errors.full_messages.each |message| %>     <li><%= message %></li>   <% end %>    </ul>    </div>    <% end %>     <div class="field"> <%= f.label :credit, "deposit amount" %><br> <%= f.text_field :credit, size: 40 %> </div> <div class="field"> <%= f.label :depotype, "select deposit method" %><br> <%= f.select :depotype, account::deposit_types, prompt: 'select deposit   method' %> </div> <div class="actions"> <%= f.submit 'deposit funds' %> </div> <% end %> 

params method in controller:

  def account_params    # params[:account]   params.require(:account).permit(:created_at, :email, :credit, :debit,    :acctbal, :depotype)    end    end 

i add previous_balance method account model:

# models/account.rb def self.previous_balance_for_user(user)   where(user_id: user.id).order(:created_at).pluck(:acctbal).first || 0.0 end 

note pluck returns values of acctbal (or nil).

with method controller can changed to:

previous_balance = account.previous_balance_for_user(current_user) @account = account.new(account_params.merge(   user_id: current_user.id,   email: current_user.email,   acctbal: previous_balance + account_params[:credit].to_f ))