This document describes how to create a custom DataMapper adapter for MongoDB. It discusses initializing the adapter, connecting to MongoDB, and implementing CRUD methods like create, read, update and delete. Methods are provided to parse DataMapper query conditions to MongoDB query formats, handle associations, and apply field and collection naming conventions. The adapter subclasses DataMapper::Adapters::AbstractAdapter and implements adapter-specific behavior while retaining compatibility with DataMapper APIs.

1. Custom DataMapper
Adapters
By Josh Moore ( )

2. About Me
• Josh Moore
• Optimis Dev
• github.com/joshsmoore
• twitter.com/codingforrent
• www.codingforrent.com
• joshsmoore@gmail.com

4. DataMapper seams to be losing
popularity why talk about it?

5. I enjoy non standard
things...

6. It still has features
ActiveRecord/ActiveModel does
not...

7. Try something new...

8. Examples
class Comment class Post
include include
DataMapper::Resource DataMapper::Resource
property :id, Serial property :id, Serial
property :body, String property :title, String
belongs_to :post has n, :comments
end end

10. Terms
term meaning
Resource A model
field A property on a model
Repository
DataMapper term for the storage engine

12. Preamble
require 'dm-core'
Just do it
module DataMapper
Just do it
module Adapters
class TestAdapter < AbstractAdapter
...
end
Just do it
const_added(:TestAdapter)
end
end

15. Symbol
Hash
def initialize(name, options)
end

16. def initialize(name, options)
Just do it
super
end

17. def initialize(name, options)
super
end

18. Connect
to your
def initialize(name, options) adapter
super
@conn = Mongo::Connection.new(
options[:host], options[:port])
@adapter = @conn[options[:database]]
end

19. def initialize(name, options)
super
@conn = Mongo::Connection.new(
Sub class of: DataMapper::Property
options[:host], options[:port])
@adapter = @conn[options[:database]]
@field_naming_convention = Proc.new do |field|
field.model.storage_name + '_' + field.name.to_s
end
Return a String
end

20. def initialize(name, options)
super
@conn = Mongo::Connection.new(
options[:host], options[:port])
@adapter = @conn[options[:database]]
@field_naming_convention = Proc.new do |field|
field.model.storage_name String (class.to_s)
+ '_' + field.name.to_s
end
@resource_naming_convention = Proc.new do |resource|
resource.downcase
end Return a String
end

21. def initialize(name, options)
super
@conn = Mongo::Connection.new(
options[:host], options[:port])
@adapter = @conn[options[:database]]
@field_naming_convention = Proc.new do |field|
field.model.storage_name + '_' + field.name.to_s
end
@resource_naming_convention = Proc.new do |resource|
resource.downcase
end
end

24. Array of Resources
def create(resources)
end
Return number of resources created

25. def create(resources)
resources.collect do |resource|
initialize_serial(resource,
@adapter[resource.class.storage_name].find.count)
fields = attributes_as_fields(
resource.attributes(:property))
@adapter[resource.class.storage_name].insert(fields)
end.size
end

26. def create(resources)
resources.collect do |resource|
initialize_serial(resource,
@adapter[resource.class.storage_name].find.count)
fields = attributes_as_fields(
resource.attributes(:property))
@adapter[resource.class.storage_name].insert(fields)
end.size
end

27. • Accepts: Hash
• Key: Sub class of: DataMapper::Property
• Value: non marshaled data
• example:
{<DataMapper::Property::String(title)> =>
"hasdf"}
def create(resources)
resources.collect do |resource|
initialize_serial(resource,
@adapter[resource.class.storage_name].find.count)
fields = attributes_as_fields(
resource.attributes(:property))
@adapter[resource.class.storage_name].insert(fields)
end.size
• Return: Hash
• Key: @field_naming_convention result
end • Value: Marshaled data
• Only values that are set
•Example:
{"post_title" => "hasdf"}

28. def create(resources)
resources.collect do |resource|
initialize_serial(resource,
@adapter[resource.class.storage_name].find.count)
fields = attributes_as_fields(
resource.attributes(:property))
@adapter[resource.class.storage_name].insert(fields)
end.size
end

29. def create(resources)
resources.collect do |resource|
initialize_serial(resource,
@adapter[resource.class.storage_name].find.count)
fields = attributes_as_fields(
resource.attributes(:property))
@adapter[resource.class.storage_name].insert(fields)
end.size
end Unless an Exception is raised the
resource will be considered saved

32. DataMapper::Query
def read(query)
end
Return a Array of Hashes
key: field name
value: unmarshed value
{field_name => value}

33. def read(query)
conditions = parse_query_conditions(query)
@adapter[query.model.storage_name].find(
conditions)
end

34. Query Structure
DataMapper::Query
#conditions
Operation
#operands
Set
Operation Condition Array
#subject
Field Association

35. Query Structure
DataMapper::Query
#conditions
Operation
#operands
Set
Operation Condition Array
#subject
Field Association
:conditions => ["updated_at > ?", Time.now]

36. Query Structure
DataMapper::Query
#conditions
Operation
#operands
Set
Operation Condition Array
#subject
Field Association
:conditions => ["updated_at > ?", Time.now]
:conditions => {:updated_at => {"$gte" => Time.now}}

37. Query Structure
DataMapper::Query
#conditions
Operation
#operands
Set
Operation Condition
Hash
Array
#subject
Field Association
:conditions => ["updated_at > ?", Time.now]
:conditions => {:updated_at => {"$gte" => Time.now}}

39. DataMapper::Query
def parse_query_conditions(query)
mongo_conditions = {}
Return a hash
mongo_conditions

40. query.conditions.operands.each do |condition|
def parse_query_conditions(query)
mongo_conditions = {}
case condition.class.to_s
when 'DataMapper::Query::Conditions::GreaterThanComparison'
mongo_conditions[condition.subject.field] =
{ "$gt" => condition.value}
when 'DataMapper::Query::Conditions::LessThanComparison'
mongo_conditions[condition.subject.field] =
{ "$lt" => condition.value}
else
mongo_conditions[condition.subject.field] =
condition.value
end mongo_conditions
end end

41. query.conditions.operands.each do |condition|
def parse_query_conditions(query)
mongo_conditions = {}
case condition.class.to_s
when 'DataMapper::Query::Conditions::GreaterThanComparison'
mongo_conditions[condition.subject.field] =
{ "$gt" => condition.value}
when 'DataMapper::Query::Conditions::LessThanComparison'
mongo_conditions[condition.subject.field] =
{ "$lt" => condition.value}
else
mongo_conditions[condition.subject.field] =
condition.value
end mongo_conditions
end end

42. query.conditions.operands.each do |condition|
def parse_query_conditions(query)
mongo_conditions = {}
case condition.class.to_s
when 'DataMapper::Query::Conditions::GreaterThanComparison'
mongo_conditions[condition.subject.field] =
{ "$gt" => condition.value}
when 'DataMapper::Query::Conditions::LessThanComparison'
mongo_conditions[condition.subject.field] =
{ "$lt" => condition.value}
else
mongo_conditions[condition.subject.field] =
condition.value
end mongo_conditions
end end

43. def parse_query_conditions(query)
mongo_conditions = {}
query.conditions.operands.each do |condition|
case condition.class.to_s
when 'DataMapper::Query::Conditions::GreaterThanComparison'
mongo_conditions[condition.subject.field] =
{ "$gt" => condition.value}
when 'DataMapper::Query::Conditions::LessThanComparison'
mongo_conditions[condition.subject.field] =
{ "$lt" => condition.value}
else
mongo_conditions[condition.subject.field] =
condition.value
end
end
mongo_conditions
end

44. Post.all(:comments => {:body => 'hi'})
comments = Comment.all(:body => 'hi')
post_ids = comments.collect { |c| c.post_id }
Post.all(:id => post_ids)

45. conditions.operands.each do |condition|
...
case condition.class.to_s
when '...InclusionComparison'
if condition.subject.instance_of?
DataMapper::Associations::OneToMany::Relationship
pk = condition.subject.parent_key.first.field
ck = condition.subject.child_key.first.name
mongo_conditions[pk] = {"$in" =>
condition.value.collect {|r| r.send(ck)}}
else
...

46. conditions.operands.each do |condition|
...
case condition.class.to_s
when '...InclusionComparison'
if condition.subject.instance_of?
DataMapper::Associations::OneToMany::Relationship
pk = condition.subject.parent_key.first.field
ck = condition.subject.child_key.first.name
mongo_conditions[pk] = {"$in" =>
condition.value.collect {|r| r.send(ck)}}
else
...

47. conditions.operands.each do |condition|
... Array of properties
case condition.class.to_s
* property - subclass of DataMapper::Property
* ex. Post#id
when '...InclusionComparison'
if condition.subject.instance_of?
DataMapper::Associations::OneToMany::Relationship
pk = condition.subject.parent_key.first.field
ck = condition.subject.child_key.first.name
mongo_conditions[pk] = {"$in" =>
condition.value.collect {|r| r.send(ck)}}
elseArray of properties
* property - subclass of DataMapper::Property
... * ex Coment#post_id

48. conditions.operands.each do |condition|
...
case condition.class.to_s
when '...InclusionComparison'
if condition.subject.instance_of?
DataMapper::Associations::OneToMany::Relationship
Array of resources
pk = condition.subject.parent_key.first.field
* [#<Comment..>, #<Comment..>,...]
ck = condition.subject.child_key.first.name
mongo_conditions[pk] = {"$in" =>
condition.value.collect {|r| r.send(ck)}}
else
...

49. conditions.operands.each do |condition|
...
case condition.class.to_s
when '...InclusionComparison'
if condition.subject.instance_of?
DataMapper::Associations::OneToMany::Relationship
pk = condition.subject.parent_key.first.field
ck = condition.subject.child_key.first.name
mongo_conditions[pk] = {"$in" =>
condition.value.collect {|r| r.send(ck)}}
else
...

50. conditions.operands.each do |condition|
...
case condition.class.to_s
when '...InclusionComparison'
if condition.subject.instance_of?
DataMapper::Associations::OneToMany::Relationship
pk = condition.subject.parent_key.first.field
ck = condition.subject.child_key.first.name
mongo_conditions[pk] = {"$in" =>
condition.value.collect {|r| r.send(ck)}}
else
...

51. Operation
• DataMapper::Query::Conditions::AndOperation
• DataMapper::Query::Conditions::OrOperation
• DataMapper::Query::Conditions::NotOperation

52. Condition
• DataMapper::Query::Conditions::EqualToComparison
• DataMapper::Query::Conditions::InclusionComparison
• DataMapper::Query::Conditions::RegexpComparison
• DataMapper::Query::Conditions::LikeComparison
• DataMapper::Query::Conditions::GreaterThanComparison
• DataMapper::Query::Conditions::LessThanComparison
• DataMapper::Query::Conditions::GreaterThanOrEqualToComparison
• DataMapper::Query::Conditions::LessThanOrEqualToComparison

53. What is not covered
• Nested Operands

54. If your backed does not
have a query language
A Array of Hashes
key: field name
value: unmarshed value
[{field_name => value}]
def read(query)
query.filter_records(records)
end

56. def delete(resources)
conditions = parse_query_conditions(resources.query)
record_count = read(resources.query).count
@adapter[resources.storage_name].remove(conditions)
record_count
end

57. DataMapper::Collection
def delete(resources)
conditions = parse_query_conditions(resources.query)
record_count = read(resources.query).count
@adapter[resources.storage_name].remove(conditions)
record_count
end Number of resources deleted (int)

58. def delete(resources)
conditions = parse_query_conditions(resources.query)
record_count = read(resources.query).count
@adapter[resources.storage_name].remove(conditions)
record_count
end
Unless an Exception is raised the
resource will be considered saved

60. def update(changes, resources)
conditions = parse_query_conditions(resources.query)
@adapter[resources.storage_name].update(conditions,
{"$set" => attributes_as_fields(changes)},
{:multi => true})
read(resources.query).count
end

61. Unmarshaled hash of changes
{<DataMapper::Property::String(title)>
=> "hasdf"}
DataMapper::Collection
def update(changes, resources)
conditions = parse_query_conditions(resources.query)
@adapter[resources.storage_name].update(conditions,
{"$set" => attributes_as_fields(changes)},
{:multi => true})
read(resources.query).count
end
Number of resources updated (int)

62. def update(changes, resources)
conditions = parse_query_conditions(resources.query)
@adapter[resources.storage_name].update(conditions,
{"$set" => attributes_as_fields(changes)},
{:multi => true})
read(resources.query).count
end
Unless an Exception is raised the
resource will be considered saved

63. Be Kind log
• DataMapper.logger

64. You can stop now
end