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Declarative transformation of MongoDB documents into mySQL tables

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See lib/example_squeal.rb for the example squeal.

To run standalone, simply make your data squeal thusly:

ruby example_squeal.rb

where the squeal script includes a require 'squealer'.

Release Notes


  • Object#import now wraps a MongoDB cursor to provide counters and timings. Only each is supported for now.
  • Progress bar and summary.


  • Object#import syntax has changed. Now import.source(collection).each rather than import.collection(collection).find({}).each. source returns a MongoDB cursor like find does. See lib/example_squeal.rb for options.


  • Object#target verifies there is a variable in scope with the same name as the table_name being targetted, it must be a Hash and must have an _id key
  • Block to Object#assign not required, infers value from source scope
  • A block returning nil now uses nil as the value to Object#assign, rather than inferring value from source scope


Squealer is for standalone operation. DO NOT use it directly from within your Ruby application. To make the DSL easy to use, we alter some core types:

  • FalseClass#to_i - You'll be storing booleans as a tinyint(1), or similar. false is 0.
  • Hash#method_missing - You prefer dot notation. JSON uses dot notation. You are importing from a data store which represents collections as arrays of hashmaps. Dot notation for navigating those collections is convenient. If you use a field name that happens to be a method on Hash you will have to use index notation. (e.g. is good, however kitten.freeze is not good. Use kitten['freeze'] instead.)
  • NilClass#each - As you are importing from schemaless repositories and you may be trying to iterate on fields that contain embedded collections, if a specific parent does not contain one of those child collections, the driver will be returning nil as the value for that field. Having NilClass#each return a [] for a nil is convenient, semantically correct in this context, and removes the need for many nil checks in the block you provide to Object#assign
  • Object - #import, #export, #target, and #assign "keywords" are provided for convenience
  • Time#to_s - As you are exporting to a SQL database, we represent your timestamp in a format that it will parse unequivocally (MongoDB stores all temporal data as a timestamp)
  • TrueClass#to_i - You'll be storing booleans as a tinyint(1), or similar. true is 1.

It is a data mapper, it doesn't use one.

Squealer doesn't use your application classes. It doesn't use your ActiveRecord models. It doesn't use mongoid (as awesome as that is), or mongomapper. It's an ETL tool. It could even be called a HRM (Hashmap-Relational-Mapper), but only in hushed tones in the corner boothes of dark pubs. It directly uses the Ruby driver for MongoDB and the Ruby driver for mySQL.

Databases supported

For now, this is specifically for MongoDB exporting to mySQL.

Deprecation Warning

Since version 1.1, the primary key value is inferred from the source document _id field based on the Object#target table_name argument matching the name of a variable holding the source document, row_id is no longer a parameter on Object#target. It will be invalid in version 1.3 and above.


The target SQL database must have no foreign keys (because it can't rely on the primary key values and referential integrity is the responsibility of the source data store or the application that uses it).

The target SQL database must use a primary key of char(24). For now, we've assumed that column name is id. Each record's id value will get the source document _id value.

It is assumed the target data will be quite denormalized - particularly that the hierarchy keys for embedded documents are flattened. This means that a document from will be exported to a record containing the id for office, the id for room and the id for box.

It is assumed no indexes are present in the target database table (performance drag). You may want to create indexes for pulling data out of the database Squealer exports to. Run a SQL DDL script on your mySQL database after squealing to add the indexes. You should drop the indexes before squealing again.

The target row is inserted, or updated if present. We are using MySQL INSERT ... UPDATE ON DUPLICATE KEY extended syntax to achieve this for now. This allows an event-driven update of exported data (e.g. through redis queues) as well as a bulk batch process.

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