Raw SQL and Queries

# Executing Raw SQL

We can execute raw sql using Avram::Database which gives us direct access to our app’s database instance. The result of the query will be a DB::ResultSet which we will later map to classes that can be easily used in our app.

posts_result_set = AppDatabase.query_all("SELECT * FROM posts", as: Post)

# Custom Queries

Since we have direct access to the database instance, we can run whatever query we want as long as it’s valid sql for the postgresql driver that Lucky uses.

Let’s create a query with a non-trivial SELECT statement that we can map to a Crystal class.

Note: This assumes we have post and user tables already created

sql = <<-SQL
  SELECT
    posts.id,
    posts.title,
    ('PREFIX: ' || posts.content) as custom_key, -- custom key for fun
    json_build_object(
      'name', users.name,
      'email', users.email
    ) as author
  FROM posts
  JOIN users
  ON users.id = posts.user_id;
SQL

# Map the Query to a Class

crystal-db comes with a powerful DB.mapping macro that makes it simple to map a database query to a class by defining the keys and types of each column.

Let’s create a ComplexPost class in our models folder and define the database mapping.

# src/models/complex_post.cr
class ComplexPost
  DB.mapping({
    id: Int64,
    title: String,
    content: {
        type: String,
        nilable: false,
        key: "custom_key"
    },
    author: JSON::Any
  })
end

# Putting it All Together

Now we can make our query and instantiate ComplexPosts from the result easily using the as method.

complex_posts = AppDatabase.query_all(sql, as: ComplexPost)

Believe it or not, that’s all it takes! complex_posts is now of the type Array(ComplexPost) and ready to be used in your pages or returned by your JSON API.

# SQL with dynamic args

If you need to pass in an external value to your query, you will need to add placeholders where your values will be inserted. This is to avoid doing string interpolation which could lead to potential security holes and SQL injection.

The placeholder for raw SQL uses the $N notation where N is the number of args being passed in. For example, if you need to pass in 2 args, you’ll use the placeholder $1 for the first arg value, and $2 for the second arg value, and so on.

sql = <<-SQL
SELECT
  posts.id,
  posts.title,
  ('PREFIX: ' || posts.content) as custom_key, -- custom key for fun
  json_build_object(
    'name', users.name,
    'email', users.email
  ) as author
FROM posts
JOIN users ON users.id = posts.user_id
WHERE posts.published_at BETWEEN ($1 AND $2);
SQL

complex_posts = AppDatabase.query_all(
  sql,
  # 4.hours.ago maps to the $1 placeholder, and 1.hour.ago maps to the $2 placeholder
  args: [4.hours.ago, 1.hour.ago],
  as: ComplexPost
)

This returns Array(ComplexPost) where the posts were published_at between 4 hours ago, and 1 hour ago.

The args argument is always an Array, even for a single argument.

# Additional Supported Methods

The query_all method is the most common since it returns an Array of rows. However, crystal-db supports many other methods you can use. Each of these are class methods called on your database class. (e.g. AppDatabase)

• exec - Used to execute SQL. Best used with Inserts, Deletes, Updates, or queries like refreshing views.
• scalar - Returns a single value like a select count, or other aggregate value
• query_all - Returns an Array of records.
• query_one - Returns a single records, or raises an exception if no record is found.
• query_one? - Same as query_one, but will return nil instead of raising an exception.

Each of these methods use the same signature for convienience.

AppDatabase.exec "REFRESH MATERIALIZED VIEW reports"
AppDatabase.scalar "SELECT SUM(amount) FROM payments WHERE user_id = $1", args: [user.id]
# The `queryable` arg is used for logging within Breeze
AppDatabase.query_all large_sql, queryable: "PostQuery", as: ComplexPost
AppDatabase.query_one "SELECT * FROM users WHERE id = -1" # raises an exception
AppDatabase.query_one? "SELECT * FROM users WHERE id = -1" # returns nil

# Escape hatch

In addition to these methods, you can also drop down to crystal-db directly by using the run method which returns the db instance to the block.

AppDatabase.run do |crystal_db|
  # call any `DB::QueryMethods` from here as needed
end