# Eno is Not an ORM [INSTALL](#installing-eno) | [TUTORIAL](#getting-started) | [EXAMPLES](examples) ## 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`: ```ruby Client.where(order_count: [1, 3, 5]) ``` And Sequel is (quite) a bit more flexible: ```ruby 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: ```ruby gem 'eno' ``` Or manually: ```bash $ gem install eno ``` ## Getting started To use eno in your code just require it: ```ruby require 'eno' ``` Alternatively, you can import it using [Modulation](https://github.com/digital-fabric/modulation): ```ruby 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: ```ruby Q { select a, b from c } ``` To turn the query into SQL, use the `#to_sql` method: ```ruby 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: ```ruby Q { select foo } #=> select foo ``` Identifiers can be qualified by using dot-notation: ```ruby Q { select foo.bar } #=> select foo.bar ``` ### Literals Literals can be specified as literals ```ruby 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`: ```ruby 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 or `!` | 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: ```ruby Q { select (a + b) & (c * d), e >= f } #=> select (a + b) and (c * d), e >= f ``` ### functions You can also use SQL functions: ```ruby 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: ```ruby 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: ```ruby Q { select c: a + b, f: d(e) } #=> select a + b as c, d(e) as f ``` Columns can be qualified using dot-notation: ```ruby 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: ```ruby 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`): ```ruby 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`: ```ruby 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: ```ruby 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](#expressions)), and can also be chained in order to mutate and further filter query: ```ruby 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: ```ruby 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: ```ruby 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.