Multimodal binding in combobox

Dear Team,
Trying to bind product details in combo box. Everything is working fine when trying to create information at first time but when editing the same then combobox selected value is blank.

**Transaction Object:**
```
{
"id": "TEST10001",
"booking_date": "09-AUG-2019 18:45:00 +05:30",
"booking_type": "LOCAL",
"type": "booking",
"customer": {
"id": "P1001",
"name": "TEST PARTY"
},
"supplier": {
"d": "1001",
"name": "TEST SUPPLIER"
},
"products": [
{
"line_id": 1,
"product": {
"product.id": "P1001",
"type": "product",
"product.description": "TEST PRODUCT 1001",
"sellingprice": 100,
"buyingprice": 70,
"_rid": "RwgzALX24FcUAAAAAAAAAA==",
"_self": "dbs/RwgzAA==/colls/RwgzALX24Fc=/docs/RwgzALX24FcUAAAAAAAAAA==/",
"_etag": "\"00000000-0000-0000-5287-27a824d301d5\"",
"_attachments": "attachments/",
"_ts": 1565776851
},
"qtydue": 100
},
{
"line_id": 2,
"product": {
"id": "P1002",
"type": "product",
"description": "TEST PRODUCT 1002",
"sellingprice": 100,
"buyingprice": 70,
"_rid": "RwgzALX24FcVAAAAAAAAAA==",
"_self": "dbs/RwgzAA==/colls/RwgzALX24Fc=/docs/RwgzALX24FcVAAAAAAAAAA==/",
"_etag": "\"00000000-0000-0000-5287-2d4b09bf01d5\"",
"_attachments": "attachments/",
"_ts": 1565776861
},
"qtydue": 500
}
],
"_rid": "RwgzANM6dFcBAAAAAAAAAA==",
"_self": "dbs/RwgzAA==/colls/RwgzANM6dFc=/docs/RwgzANM6dFcBAAAAAAAAAA==/",
"_etag": "\"00000000-0000-0000-528d-b11613a201d5\"",
"_attachments": "attachments/",
"_ts": 1565779659
}

```

**Product Master Object:**
```
{
"id": "P1001",
"type": "product",
"description": "TEST PRODUCT 1001",
"sellingprice": 100,
"buyingprice": 70,
"_rid": "RwgzALX24FcUAAAAAAAAAA==",
"_self": "dbs/RwgzAA==/colls/RwgzALX24Fc=/docs/RwgzALX24FcUAAAAAAAAAA==/",
"_etag": "\"00000000-0000-0000-5287-27a824d301d5\"",
"_attachments": "attachments/",
"_ts": 1565776851
}
```