fix(Purchase): validation of form.

tienda_ilusion/don_confiao/frontend/don-confiao/src/components/Purchase.vue

@@ -1,6 +1,6 @@
 <template>
 <v-container>
-  <v-form ref="form" v-model="valid">
+  <v-form ref="purchase" v-model="valid" >
     <v-row>
       <v-col>
         <v-autocomplete
@@ -123,6 +123,8 @@
             item-title="text"
             item-value="value"
             label="Pago en"
+            :rules="[rules.required]"
+            required
           ></v-select>
           <v-btn @click="openCasherModal" v-if="purchase.payment_method === 'CASH'">Calcular Devuelta</v-btn>
           <CasherModal :total_purchase="calculateTotal" ref="casherModal"</CasherModal>
@@ -276,9 +278,10 @@
        return line.unit_price * line.quantity;
      },
      async submit() {
-       if (this.$refs.form.validate()) {
-           const hasInvalidQuantity = this.purchase.saleline_set.some(line => line.quantity <= 0);
-           if (hasInvalidQuantity) {
+       this.$refs.purchase.validate();
+       if (this.valid) {
+         const hasInvalidQuantity = this.purchase.saleline_set.some(line => line.quantity <= 0);
+         if (hasInvalidQuantity) {
                this.errorMessage = 'La cantidad de cada línea de compra debe ser mayor que cero.';
 
                console.log(this.errorMessage);