Eno is Not an ORM

What is Eno?

Eno is an experimental Ruby gem for working with SQL databases. Eno provides tools for writing SQL queries using plain Ruby and specifically for querying PostgreSQL and SQLite databases.

Eno provides the following features:

Compose SELECT statements using plain Ruby syntax
Create arbitrarily complex WHERE clauses
Support for common table expressions (CTE) and joins
Compose queries and sub-queries
Create parametric queries using context variables
Reusable queries can be further refined and mutated

What is it good for?

So why would anyone want to compose queries in Ruby instead of in plain SQL? That's actually a very good question. Libraries like ActiveRecord and Sequel already provide tools for querying relational databases. There's usage patterns like ActiveRecord's where:

Client.where(order_count: [1, 3, 5])

And Sequel is (quite) a bit more flexible:

Client.where { order_count > 10 }

But both stumble when it comes to putting together more complex queries. ActiveRecord queries in particular aren't really composable, making it actually easier to filter and manipulate records inside your app code than in your database.

With both ActiveRecord and Sequel you'll need to eventually provide snippets of literal SQL. This is time-consuming, prevents your queries from being composable and makes it easy to expose your app to SQL injection.

Installing eno

Using bundler:

gem 'eno'

Or manually:

$ gem install eno

Getting started

To use eno in your code just require it:

require 'eno'

Alternatively, you can import it using Modulation:

Eno = import('eno')

Putting together queries

Eno makes it easy to compose SQL queries using plain Ruby syntax. It takes care of formatting table and column identifiers and literals, and allows you to compose multiple queries into a single SELECT statement.

To compose a query use the Kernel#Q method, providing a block in which the query is built:

Q {
  select a, b
  from c
}

To turn the query into SQL, use the #to_sql method:

Q {
  select a, b
  from c
}.to_sql #=> "select a, b from c"

Expressions

Eno lets you build arbitrarily complex expressions once inside the query block. You can freely mix identifiers and literals, use most operators (with certain caveats) and make function calls.

Identifiers

An identifier is referenced simply using its name:

Q {
  select foo
} #=> select foo

Identifiers can be qualified by using dot-notation:

Q {
  select foo.bar
} #=> select foo.bar

Literals

Literals can be specified as literals

Q {
  select x * 10
} #=> select x * 10

However, if the first argument of an expression is a literal, it will need to be wrapped in a call to #_q:

Q {
  select _q(2) + 2
} #=> select 2 + 2

Operators

Eno supports the following mathematical operators:

operator	description
`+`	addition
`-`	subtraction
`*`	multiplication
`/`	division
`%`	modulo (remainder)

Logical operators are supported using the following operators:

operator	description
`&`	logical and
`\	`
`!`	logical not

The following comparison operators are supported:

operator	description
`==`	equal
`!=`	not equal
`<`	less than
`>`	greater than
`<=`	less than or equal
`>=`	greater than or equal

An example involving multiple operators:

Q {
  select (a + b) & (c * d), e >= f
} #=> select (a + b) and (c * d), e >= f

functions

You can also use SQL functions:

Q {
  select user_id, max(score)
  from exams
  group_by user_id
}

SQL clauses

Eno supports the following clauses:

Select

The #select method is used to specify the list of selected expressions for a select statement. The select method accepts a list of expressions:

Q { select a, b + c, d.as(e) } #=> select a, b + c, d as e

The #select method can also accept a hash mapping aliases to expressions:

Q { select c: a + b, f: d(e) } #=> select a + b as c, d(e) as f

Columns can be qualified using dot-notation:

Q { select a.b, c.d.e } #=> select a.b, c.d.e

Note: if #select is not called within a query block, a select * is assumed:

Q { from mytable } #=> select * from mytable

From

The #from method is used to specify one or more sources for the query. Usually this would be a table name, a subquery, a CTE name (specified using #with):

Q { from a, b, c } #=> select * from a, b, c
Q { from a.as b } #=> select * from a as b

Subqueries can also be used in #from:

Q {
  select sum(foo.score)
  from Q { select * from scores }.as(foo)
} #=> select sum(foo.score) from (select score from scores) as foo

Where

The #where method is used to specify a record filter:

Q {
  from users
  where name == 'John Doe' & age > 30
} #=> select * from users where (name = 'John Doe') and (age > 30)

Where clauses can be of arbitrary complexity (as shown above), and can also be chained in order to mutate and further filter query:

query = Q {
  from users
  where state == 'CA'
}
query.where { age >= 25 } #=> select * from users where (state = 'CA') and (age >= 25)

Hooking up Eno to your database

In and of itself, Eno is just an engine for building SQL queries. To actually run your queries, you'll need to hook Eno to your database. Here's an example of how to open a connection to a PostgreSQL database and then easily issue queries to it:

require 'pg'

DB = PG.connect(host: '/tmp', dbname: 'myapp', user: 'myuser')
def DB.q(**ctx, &block)
  query(**ctx, &block).to_a
end

# issue a query
DB.q {
  from users
  select 
}

Another way to issue queries is by defining methods on Eno::Query:

def Eno::Query.each(**ctx, &block)
  DB.query(to_sql(**ctx)).each(&block)
end

Roadmap

Eno is intended as a complete solution for eventually expressing any SQL query in Ruby (including INSERT, UPDATE and DELETE and ALTER TABLE statements).

In the future, Eno could be used to manipulate queries in other ways:

EXPLAIN your queries.
Introspect different parts of a query (for example look at results of subqueries or CTE's).
Transform CTE's into subqueries (for example to overcome optimization boundaries).
Create views from queries.
Compose data manipulation statements using SELECT subqueries.