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Docs on how to use ToField to query custom tuples in IN clause #1863

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65 changes: 65 additions & 0 deletions Guide/database.markdown
Original file line number Diff line number Diff line change
Expand Up @@ -334,6 +334,71 @@ do
|> map (\(Only uuid) -> Id uuid :: Id Project)
```

### `IN` Clause Queries with Multiple Parameters

At times, you may need to utilize the IN clause in SQL queries with multiple parameters. This approach differs from searching for records that match a single parameter, such as an ID. Instead, you might search for records that meet a combination of two parameters, like `project_id` and `user_id`.

Consider a Project record, which includes a project type enum and the number of participants:

```
# Schema.sql
CREATE TYPE project_type AS ENUM ('project_type_ongoing', 'project_type_not_started', 'project_type_finished');

CREATE TABLE projects (
id UUID DEFAULT uuid_generate_v4() PRIMARY KEY NOT NULL,
project_type project_type NOT NULL,
participants TEXT NOT NULL
);
```

In our Fixtures.sql, we can insert some sample data:

```sql
ALTER TABLE public.projects DISABLE TRIGGER ALL;

INSERT INTO public.projects (id, project_type, participants) VALUES ('e767f087-d1b9-42ea-898e-c2a2bf39999e', 'project_type_ongoing', '2');
INSERT INTO public.projects (id, project_type, participants) VALUES ('429177d7-f425-4c5e-b379-5e1a0f72bfb5', 'project_type_ongoing', '2');
INSERT INTO public.projects (id, project_type, participants) VALUES ('84825fa3-2cce-4b4a-872b-81fe554e2076', 'project_type_not_started', '3');
INSERT INTO public.projects (id, project_type, participants) VALUES ('687e32f5-8e8b-4a6d-b4a7-ead9d8a39f91', 'project_type_finished', '2');


ALTER TABLE public.projects ENABLE TRIGGER ALL;
```

Now, let's say we want to retrieve all projects that are of type Ongoing and have 1 participant, and the Finished ones with 2 participants. The filterWhere function is not suitable here since we need to query by two parameters.

First, we must implement ToField to allow using a tuple of `(ProjectType, Int)` as a parameter:

```haskell
-- Web/Types.hs
import Database.PostgreSQL.Simple.ToField
import Data.ByteString.Builder (char8)


instance ToField (ProjectType, Int) where
toField = serializeProjectTypeAndInt

serializeProjectTypeAndInt :: (ProjectType, Int) -> Action
serializeProjectTypeAndInt (projectType, participants) = Many
[ Plain (char8 '(')
, toField projectType
, Plain (char8 ',')
, toField $ show participants
, Plain (char8 ')')
]
```

Then, in our controller, we can execute the query:

```haskell
-- Web/Controller/Projects.hs
action ProjectsAction = do
let pairs = [(ProjectTypeOngoing, 1 :: Int), (ProjectTypeFinished, 2)]
projects :: [Project] <- sqlQuery "SELECT * FROM projects WHERE (project_type, participants) IN ?" (Only $ In pairs)
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render IndexView { .. }
```

### Scalar Results

The [`sqlQuery`](https://ihp.digitallyinduced.com/api-docs/IHP-ModelSupport.html#v:sqlQuery) function always returns a list of rows as the result. When the result of your query is a single value (such as an integer or string) use [`sqlQueryScalar`](https://ihp.digitallyinduced.com/api-docs/IHP-ModelSupport.html#v:sqlQueryScalar):
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