# Inject Values Using Hints

Sometimes an entity contains a property that is not mapped to a simple column, but instead holds some value fetched from another (joined) table or a value generated by query. For example, such property can holds some count or the result of some math operation, or value from other table.

We will use example where property `SentMessage.content` should be populated with value of `Message.ctx` (string value). Here is the T-SQL query that joins two tables and selects everything what is needed:

```sql
    select $C{sm.*}, $C{msg.ctx} from $T{SentMessage sm}
    join ($T{Message msg}) on ($msg.id=$sm.refId) where ...
```

Since `msg.ctx` string is not part of `SentMessage`, we can only call `DbOomQuery#list(SentMessage.class, String.class)` to receive data. But, this would be painful! The returned result for above T-SQL query is `List<Object[]>`, and in order to populate each `SentMessage.content` we would need to:

1. create a new `List<SentMessage>`.
2. iterate returned list of `Object[]`.
3. for each list element, cast array elements to `SentMessage` and

   `String`.
4. manually set property `content` of iterated `SentMessage` instance

   with string value.
5. add such prepared `SentMessage` to the new list.

Not only that it is unnecessary complicated, but also some additional memory is allocated to hold both lists in the same time.

### Using hints

Fortunately, *DbOom* framework comes with *hints*;) As explained elsewhere, hints can be used to instruct how returned values should be injected one into another.

So what would be the injection hint in our example? Its easy: for each row, please inject `msg.ctx` into `sm.content`. In the other words, we can say the following: map first columns of returned result into `sm` (`SentMessage`) and the last column put into `sm.content`.

Here is how to specify this hint in Java:

```java
    DbOomQuery q = query(sql(
            "select $C{sm.*}, $C{msg.ctx} as content from $T{SentMessage sm} " +
            "join ($T{Message msg}) on ($msg.id=$sm.refId) where..."));
    // set parameters
    List<SentMessage> list =
        q.withHints("sm", "sm.content").
            list(SentMessage.class, String.class);
```

And that is all! Here is how to read this in English: each record from resultset map into `SentMessage` and `String`, and name the first instance as "`sm`", while the second value, the string value, inject into `SentMessage`instance.

There is just one caveat - column name (specified with \\'`AS`\\' keyword) must match the property name.

Also note that returned value is `List<SentMessage>`, where each `SentMessage` has populated `content` property; so everything is ready for you :)