The document provides information on schema design in MongoDB, including: 1) Discussing benefits of modeling data independently of logic using a document-based model similar to relational databases. 2) Demonstrating examples of modeling data including embedding documents and linking collections to extend the schema. 3) Covering additional topics like indexing, querying, aggregation techniques like map-reduce and grouping.

1. Schema Design
Bernie Hackett
bernie@10gen.com

2. Topics
Introduction
• Basic Data Modeling
• Manipulating Data
• Evolving a schema
Common patterns
• Single table inheritance
• One-to-Many & Many-to-Many
• Trees
• Queues

3. So why model data?
http://www.flickr.com/photos/42304632@N00/493639870/

4. Benefits of relational
• Before relational
• Data and Logic combined
• After relational
• Separation of concerns
• Data modeled independent of logic
• Logic freed from concerns of data design
• MongoDB continues this separation

5. Normalization
Goals
• Avoid anomalies when inserting, updating or
deleting
• Minimize redesign when extending the schema
• Make the model informative to users
• Avoid bias toward a particular query
In MongoDB
• Similar goals apply
• The rules are different

6. Relational made normalized
data look like this

7. Document databases make
normalized data look like this

8. Terminology
RDBMS MongoDB
Table Collection
Row(s) JSON Document
Index Index
Join Embedding & Linking
Partition Shard
Partition Key Shard Key

9. DB Considerations
How can we manipulate Access Patterns?
this data?
• Dynamic Queries • Read / Write Ratio
• Secondary Indexes • Types of updates
• Atomic Updates • Types of queries
• Map Reduce • Data life-cycle
Further Considerations
• No Joins
• Document writes are atomic

10. So today’s example will use...

11. Design Session
Design documents that simply map to
your application
> post = { author: "Hergé",
date: new Date(),
text: "Destination Moon",
tags: [ "comic",
"adventure" ] }
> db.post.save(post)

12. Find the document
> db.posts.find()
{ _id: ObjectId("4c4ba5c0672c685e5e8aabf3"),
author: "Hergé",
date: "Sat Jul 24 2010 19:47:11 GMT‐0700 (PDT)",
text: "Destination Moon",
tags: [ "comic", "adventure" ]
}
Notes:
• ID must be unique, but can be anything you’d like
• MongoDB will generate a default ID if one is not
supplied

13. Add and index, find via Index
Secondary index for "author"
// 1 means ascending, ‐1 means descending
> db.posts.ensureIndex( {author: 1 } )
> db.posts.find( { author: 'Hergé' } )
{ _id: ObjectId("4c4ba5c0672c685e5e8aabf3"),
date: "Sat Jul 24 2010 19:47:11 GMT‐0700 (PDT)",
author: "Hergé",
... }

14. Verifying indexes exist
> db.posts.getIndexes()
// Index on ID
{ name: "_id_",
ns: "test.posts",
key: { "_id" : 1 } }
// Index on author
{ _id: ObjectId("4c4ba6c5672c685e5e8aabf4"),
ns: "test.posts",
key: { "author" : 1 },
name: "author_1" }

15. Examine the query plan
> db.blogs.find( { author: 'Hergé' } ).explain()
{
"cursor" : "BtreeCursor author_1",
"nscanned" : 1,
"nscannedObjects" : 1,
"n" : 1,
"millis" : 5,
"indexBounds" : {
"author" : [
[
"Hergé",
"Hergé"
]
]
}
}

16. Query operators
Conditional operators:
$ne, $in, $nin, $mod, $all, $size, $exists, $type,
$lt, $lte, $gt, $gte
// find posts with any tags
> db.posts.find( { tags: { $exists: true } } )

17. Query operators
Conditional operators:
$ne, $in, $nin, $mod, $all, $size, $exists, $type,
$lt, $lte, $gt, $gte
// find posts with any tags
> db.posts.find( { tags: { $exists: true } } )
Regular expressions:
// posts where author starts with h
> db.posts.find( { author: /^h/i } )

18. Query operators
Conditional operators:
$ne, $in, $nin, $mod, $all, $size, $exists, $type,
$lt, $lte, $gt, $gte
// find posts with any tags
> db.posts.find( { tags: { $exists: true } } )
Regular expressions:
// posts where author starts with h
> db.posts.find( { author: /^h/i } )
Counting:
// number of posts written by Hergé
> db.posts.find( { author: "Hergé" } ).count()

19. Extending the Schema
> new_comment = { author: "Bernie",
date: new Date(),
text: "great book" }
> db.posts.update(
{ text: "Destination Moon" },
{ '$push': { comments: new_comment },
'$inc': { comments_count: 1 } } )

20. Extending the Schema
{ _id : ObjectId("4c4ba5c0672c685e5e8aabf3"),
author : "Hergé",
date : "Sat Jul 24 2010 19:47:11 GMT‐0700 (PDT)",
text : "Destination Moon",
tags : [ "comic", "adventure" ],
comments : [
{
author : "Bernie",
date : "Sat Jul 24 2010 20:51:03 GMT‐0700 (PDT)",
text : "great book"
}
],
comments_count: 1
}

21. Extending the Schema
// create index on nested documents:
> db.posts.ensureIndex( { "comments.author": 1 } )
> db.posts.find( { "comments.author": "Bernie" } )

22. Extending the Schema
// create index on nested documents:
> db.posts.ensureIndex( { "comments.author": 1 } )
> db.posts.find( { "comments.author": "Bernie" } )
// find last 5 posts:
> db.posts.find().sort( { date: ‐1 } ).limit(5)

23. Extending the Schema
// create index on nested documents:
> db.posts.ensureIndex( { "comments.author": 1 } )
> db.posts.find( { "comments.author": "Bernie" } )
// find last 5 posts:
> db.posts.find().sort( { date: ‐1 } ).limit(5)
// most commented post:
> db.posts.find().sort( { comments_count:
‐1 } ).limit(1)
When sorting, check if you need an index

24. Watch for full table scans
> db.blogs.find( { text: 'Destination
Moon' } ).explain()
{
"cursor" : "BasicCursor",
"nscanned" : 1,
"nscannedObjects" : 1,
"n" : 1,
"millis" : 0,
"indexBounds" : {
}
}

25. Map Reduce

26. Map reduce : count tags
mapFunc = function () {
this.tags.forEach( function( z ) { emit( z, { count:
1 } ); } );
}
reduceFunc = function( k, v ) {
var total = 0;
for ( var i = 0; i < v.length; i++ ) {
total += v[i].count;
}
return { count: total };
}
res = db.posts.mapReduce( mapFunc, reduceFunc )
>db[res.result].find()
{ _id : "comic", value : { count : 1 } }
{ _id : "adventure", value : { count : 1 } }

27. Group
• Equivalent to a Group By in SQL
• Specify the attributes to group the data
• Process the results in a Reduce function

28. Group - Count post by Author
cmd = { key: { "author": true },
initial: { count: 0 },
reduce: function(obj, prev) {
prev.count++;
},
};
result = db.posts.group(cmd);
[
{
"author" : "Hergé",
"count" : 1
},
{
"author" : "Kyle",
"count" : 3
}
]

29. Review
So Far:
- Started out with a simple schema
- Queried Data
- Evolved the schema
- Queried / Updated the data some more

30. Inheritance

31. Single Table Inheritance - RDBMS
shapes table
id type area radius d length width
1 circle 3.14 1
2 square 4 2
3 rect 10 5 2

32. Single Table Inheritance -
MongoDB
> db.shapes.find()
{ _id: "1", type: "circle", area: 3.14, radius: 1 }
{ _id: "2", type: "square", area: 4, d: 2 }
{ _id: "3", type: "rect", area: 10, length: 5, width: 2 }

33. Single Table Inheritance -
MongoDB
> db.shapes.find()
{ _id: "1", type: "circle", area: 3.14, radius: 1 }
{ _id: "2", type: "square", area: 4, d: 2 }
{ _id: "3", type: "rect", area: 10, length: 5, width: 2 }
// find shapes where radius > 0
> db.shapes.find( { radius: { $gt: 0 } } )

34. Single Table Inheritance -
MongoDB
> db.shapes.find()
{ _id: "1", type: "circle", area: 3.14, radius: 1 }
{ _id: "2", type: "square", area: 4, d: 2 }
{ _id: "3", type: "rect", area: 10, length: 5, width: 2 }
// find shapes where radius > 0
> db.shapes.find( { radius: { $gt: 0 } } )
// create index
> db.shapes.ensureIndex( { radius: 1 } )

35. One to Many
One to Many relationships can specify
• degree of association between objects
• containment
• life-cycle

36. One to Many
- Embedded Array / Array Keys
- slice operator to return subset of array
- some queries harder
e.g find latest comments across all documents
blogs: {
author : "Hergé",
date : "Sat Jul 24 2010 19:47:11 GMT‐0700 (PDT)",
comments : [
{
author : "Bernie",
date : "Sat Jul 24 2010 20:51:03 GMT‐0700 (PDT)",
text : "great book"
}
] }

37. One to Many
- Embedded tree
- Single document
- Natural
- Hard to query
blogs: {
author : "Hergé",
date : "Sat Jul 24 2010 19:47:11 GMT‐0700 (PDT)",
comments : [
{
author : "Bernie",
date : "Sat Jul 24 2010 20:51:03 GMT‐0700 (PDT)",
text : "great book",
replies: [ { author : “James”, ... } ]
}
] }

38. One to Many
- Normalized (2 collections)
- most flexible
- more queries
blogs: {
author : "Hergé",
date : "Sat Jul 24 2010 19:47:11 GMT‐0700 (PDT)",
comments : [
{ comment : ObjectId(“1”) }
] }
comments : { _id : “1”,
author : "James",
date : "Sat Jul 24 2010 20:51:03 ..." }

39. One to Many - patterns
- Embedded Array / Array Keys
- Embedded Array / Array Keys
- Embedded tree
- Normalized

40. Many - Many
Example:
- Product can be in many categories
- Category can have many products

41. Many - Many
products:
{ _id: ObjectId("10"),
name: "Destination Moon",
category_ids: [ ObjectId("20"), ObjectId("30") ] }

42. Many - Many
products:
{ _id: ObjectId("10"),
name: "Destination Moon",
category_ids: [ ObjectId("20"), ObjectId("30") ] }
categories:
{ _id: ObjectId("20"),
name: "adventure",
product_ids: [ ObjectId("10"), ObjectId("11"),
ObjectId("12") ] }

43. Many - Many
products:
{ _id: ObjectId("10"),
name: "Destination Moon",
category_ids: [ ObjectId("20"), ObjectId("30") ] }
categories:
{ _id: ObjectId("20"),
name: "adventure",
product_ids: [ ObjectId("10"), ObjectId("11"),
ObjectId("12") ] }
//All categories for a given product
> db.categories.find( { product_ids: ObjectId("10") } )

44. Alternative
products:
{ _id: ObjectId("10"),
name: "Destination Moon",
category_ids: [ ObjectId("20"), ObjectId("30") ] }
categories:
{ _id: ObjectId("20"),
name: "adventure" }

45. Alternative
products:
{ _id: ObjectId("10"),
name: "Destination Moon",
category_ids: [ ObjectId("20"), ObjectId("30") ] }
categories:
{ _id: ObjectId("20"),
name: "adventure" }
// All products for a given category
> db.products.find( { category_ids: ObjectId("20") } )

46. Alternative
products:
{ _id: ObjectId("10"),
name: "Destination Moon",
category_ids: [ ObjectId("20"), ObjectId("30") ] }
categories:
{ _id: ObjectId("20"),
name: "adventure" }
// All products for a given category
> db.products.find( { category_ids: ObjectId("20") } )
// All categories for a given product
product = db.products.find(_id : some_id)
> db.categories.find( { _id : { $in :
product.category_ids } } )

47. Trees
Full Tree in Document
{ comments: [
{ author: "Bernie", text: "...",
replies: [
{author: "James", text: "...",
replies: [ ] }
] }
]
}
Pros: Single Document, Performance, Intuitive
Cons: Hard to search, Partial Results, 16MB limit

48. Trees
Parent Links
- Each node is stored as a document
- Contains the id of the parent
Child Links
- Each node contains the id’s of the children
- Can support graphs (multiple parents / child)

49. Array of Ancestors
- Store all Ancestors of a node
{ _id: "a" }
{ _id: "b", ancestors: [ "a" ], parent: "a" }
{ _id: "c", ancestors: [ "a", "b" ], parent: "b" }
{ _id: "d", ancestors: [ "a", "b" ], parent: "b" }
{ _id: "e", ancestors: [ "a" ], parent: "a" }
{ _id: "f", ancestors: [ "a", "e" ], parent: "e" }

50. Array of Ancestors
- Store all Ancestors of a node
{ _id: "a" }
{ _id: "b", ancestors: [ "a" ], parent: "a" }
{ _id: "c", ancestors: [ "a", "b" ], parent: "b" }
{ _id: "d", ancestors: [ "a", "b" ], parent: "b" }
{ _id: "e", ancestors: [ "a" ], parent: "a" }
{ _id: "f", ancestors: [ "a", "e" ], parent: "e" }
//find all descendants of b:
> db.tree2.find( { ancestors: 'b' } )
//find all direct descendants of b:
> db.tree2.find( { parent: 'b' } )

51. Array of Ancestors
- Store all Ancestors of a node
{ _id: "a" }
{ _id: "b", ancestors: [ "a" ], parent: "a" }
{ _id: "c", ancestors: [ "a", "b" ], parent: "b" }
{ _id: "d", ancestors: [ "a", "b" ], parent: "b" }
{ _id: "e", ancestors: [ "a" ], parent: "a" }
{ _id: "f", ancestors: [ "a", "e" ], parent: "e" }
//find all descendants of b:
> db.tree2.find( { ancestors: 'b' } )
//find all direct descendants of b:
> db.tree2.find( { parent: 'b' } )
//find all ancestors of f:
> ancestors = db.tree2.findOne( { _id: 'f' } ).ancestors
> db.tree2.find( { _id: { $in : ancestors } )

52. Trees as Paths
Store hierarchy as a path expression
- Separate each node by a delimiter, e.g. "/"
- Use text search for find parts of a tree
{ comments: [
{ author: "Bernie", text: "initial post",
path: "/" },
{ author: "Jim", text: "jim’s comment",
path: "/jim" },
{ author: "Bernie", text: "Bernie’s reply to Jim",
path : "/jim/bernie"} ] }
// Find the conversations Jim was a part of
> db.posts.find( { path: /jim/i } )

53. Queue
• Need to maintain order and state
• Ensure that updates to the queue are atomic
{ inprogress: false,
priority: 1,
...
}

54. Queue
• Need to maintain order and state
• Ensure that updates to the queue are atomic
{ inprogress: false,
priority: 1,
...
}
// find highest priority job and mark as in‐progress
job = db.jobs.findAndModify( {
query: { inprogress: false },
sort: { priority: ‐1 },
update: { $set: {inprogress: true,
started: new Date() } },
new: true } )

55. Summary
Schema design is different in MongoDB
Basic data design principals stay the same
Focus on how the apps manipulates data
Rapidly evolve schema to meet your requirements
Enjoy your new freedom, use it wisely :-)

56. download at mongodb.org
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