diff --git a/lib/invoice.rb b/lib/invoice.rb
new file mode 100644
index 0000000..7354f48
--- /dev/null
+++ b/lib/invoice.rb
@@ -0,0 +1,95 @@
+require 'sequel'
+require 'securerandom'
+require 'json'
+require 'bigdecimal'
+
+class Invoice < Sequel::Model(:invoices)
+  CURRENCIES = %w[USD EUR GEL].freeze
+
+  plugin :json_serializer
+
+  def items_array
+    raw = self[:items]
+    raw.is_a?(String) ? JSON.parse(raw) : raw
+  end
+
+  def items_array=(arr)
+    self[:items] = Sequel.pg_jsonb_wrap(arr)
+  rescue NoMethodError
+    self[:items] = arr.to_json
+  end
+
+  def paid?
+    !paid_at.nil?
+  end
+
+  def status
+    paid? ? 'paid' : 'pending'
+  end
+
+  def total
+    BigDecimal(subtotal.to_s)
+  end
+
+  def total_gel
+    (total * BigDecimal(gel_rate.to_s)).round(2)
+  end
+
+  class << self
+    def allocate_number(year = Date.today.year)
+      prefix = "INV-#{year}-"
+      last   = where(Sequel.like(:number, "#{prefix}%"))
+                 .order(Sequel.desc(:number))
+                 .get(:number)
+      n = last ? last.split('-').last.to_i + 1 : 1
+      format("#{prefix}%04d", n)
+    end
+
+    def build(params)
+      items = normalize_items(params.fetch(:items))
+      subtotal = items.sum { |i| BigDecimal(i['qty'].to_s) * BigDecimal(i['unit_price'].to_s) }
+
+      Invoice.new(
+        uuid:           SecureRandom.uuid,
+        number:         allocate_number,
+        client_name:    params.fetch(:client_name).to_s.strip,
+        client_email:   params.fetch(:client_email).to_s.strip,
+        client_address: params[:client_address].to_s.strip,
+        currency:       normalize_currency(params.fetch(:currency)),
+        gel_rate:       BigDecimal(params.fetch(:gel_rate).to_s),
+        subtotal:       subtotal,
+        items:          items.to_json,
+        issued_on:      parse_date(params[:issued_on]) || Date.today,
+        due_on:         parse_date(params[:due_on])    || (Date.today + 14),
+        notes:          params[:notes].to_s.strip,
+        pdf_key:        '',
+        created_at:     Time.now.utc
+      )
+    end
+
+    private
+
+    def normalize_currency(c)
+      c = c.to_s.upcase
+      CURRENCIES.include?(c) or raise ArgumentError, "Unsupported currency: #{c}"
+      c
+    end
+
+    def normalize_items(raw)
+      Array(raw).filter_map do |row|
+        desc = row['description'].to_s.strip
+        next if desc.empty?
+        qty  = BigDecimal(row['qty'].to_s.empty?        ? '0' : row['qty'].to_s)
+        unit = BigDecimal(row['unit_price'].to_s.empty? ? '0' : row['unit_price'].to_s)
+        { 'description' => desc, 'qty' => qty.to_s('F'), 'unit_price' => unit.to_s('F') }
+      end
+    end
+
+    def parse_date(s)
+      return nil if s.nil? || s.to_s.strip.empty?
+      Date.parse(s.to_s)
+    rescue ArgumentError
+      nil
+    end
+  end
+end