ActiveRecord::StatementInvalid: PG::UndefinedFunction: ERROR: could not identify an equality operator for type json

ActiveRecord::StatementInvalid: PG::UndefinedFunction: ERROR: could not identify an equality operator for type json

Explanation:
```
The reason behind this, is that in PostgreSQL (up to 9.3) there is no equality operator defined for json (i.e. val1::json = val2::json will always throw this exception) -- in 9.4 there will be one for the jsonb type.

One workaround is, you can cast your json field to text. But that won't cover all json equalitions. f.ex. {"a":1,"b":2} should be equal to {"b":2,"a":1}, but won't be equal if casted to text.
```

Possible ways to fix:

1) Migrate to Postgresql 9.4 and use .jsonb type
2) Postgresql 9.3 ways to solve:

2.1) if you have a primary key for that table -- which should be - then you can use the DISTINCT ON (<expressions>) form:
```
u.profiles.select("DISTINCT ON (profiles.id) profiles.*")  
```

* But:
```
DISTINCT ON ( expression [, ...] ) keeps only the first row of each set of rows where the given expressions evaluate to equal. [...] Note that the "first row" of each set is unpredictable unless ORDER BY is used to ensure that the desired row appears first. [...] The DISTINCT ON expression(s) must match the leftmost ORDER BY expression(s).
```
So need to use "profiles.id" a the first option for ORDER BY (which can change order you need)

But you can also try to fix it using subquery:
```
SELECT *
FROM  (
    SELECT DISTINCT ON (address_id) *
    FROM   purchases
    WHERE  product_id = 1
    ORDER  BY address_id, purchased_at DESC -- to get "latest" row per addr.
    ) p
ORDER BY purchased_at DESC;
```

2.2) use Group instead of Distinct
```
.group("profiles.id")
```