This is the second post in a short series on the topic. The last post looked at the tables GROUPS and TEAMS in the OpenFootball relational database schema. There is also the table GROUPS_TEAMS, usually known as a link table, which, ahem, "relates" the GROUPS and TEAMS table. GROUPS_TEAMS has the following schema:

CREATE TABLE IF NOT EXISTS "groups_teams" (
  "id" integer PRIMARY KEY AUTOINCREMENT NOT NULL, 
  "group_id" integer NOT NULL, 
  "team_id" integer NOT NULL, 
  "created_at" datetime NOT NULL,
  "updated_at" datetime NOT NULL
);

A row in GROUPS_TEAMS with group_id of XXX and team_id of YYY means that the team represented by team_id YYY is in the group with group_id XXX.

Let's modify the Smalltalk class OFGroup to handle the linkage, by adding the inst-var 'teams' and creating accessors for it.

OFObject subclass: #OFGroup
  instanceVariableNames: 'eventId title pos teams'
  classVariableNames: ''
  package: 'OpenFootball'

Next, modify the mapping for OFGroup in OFDescriptorSystem:

classModelForOFGroup: aClassModel
  self virtualClassModelForOFObject: aClassModel.
  aClassModel newAttributeNamed: #eventId type: Integer.
  aClassModel newAttributeNamed: #title type: String.
  aClassModel newAttributeNamed: #pos type: Integer.
  "Next item is for linking OFGroup with OFTeam."
  aClassModel newAttributeNamed: #teams collectionOf: OFTeam.

descriptorForOFGroup: aDescriptor
  | t | 
  t := self tableNamed: 'GROUPS'.
  aDescriptor table: t.
  self virtualDescriptorForOFObject: aDescriptor with: t.
  (aDescriptor newMapping: DirectMapping)
    from: #eventId
    type: Integer
    to: (t fieldNamed: 'event_id').
  (aDescriptor newMapping: DirectMapping)
    from: #title
    type: String
    to: (t fieldNamed: 'title').
  (aDescriptor newMapping: DirectMapping)
    from: #pos
    type: Integer
    to: (t fieldNamed: 'pos'.
  "Next item is for linking OFGroup with OFTeam."
  (aDescriptor newMapping: ManyToManyMapping)
    attributeName: #teams.

"No change to #tableForGROUPS:."

It is now necessary to add the table GROUPS_TEAMS to OFDescriptorSystem:

tableForGROUPS_TEAMS: aTable
  | gid tid |
  self virtualTableForOFObject: aTable.
  gid := aTable createFieldNamed: 'group_id' type: platform integer.
  aTable addForeignKeyFrom: gid to: ((self tableNamed: 'GROUPS') fieldNamed: 'id').
  tid := aTable createFieldNamed: 'team_id' type: platform integer.
  aTable addForeignKeyFrom: tid to: ((self tableNamed: 'TEAMS') fieldNamed: 'id').

Now let's fetch the OFGroup instances with their linked OFTeam instances.

| vh |
Transcript clear.
OFDatabase dbFileName: 'wc2018.db'
  evaluate: [ :db |
    db session accessor logging: true. "This shows the generated SQL."
    vh := String streamContents: [ :str | 
      (db session read: OFGroup) do: [ :ea | 
        str nextPutAll: ea title; nextPut: Character cr.
        ea teams do: [ :team | 
          str nextPutAll: '- ', team title; nextPut: Character cr ]]]].
vh

The above snippet produces the following output:

Group A
- Egypt
- Russia
- Saudi Arabia
- Uruguay
<some output omitted>
Group H
- Senegal
- Japan
- Poland
- Colombia

In the snippet, logging is enabled, and the SQL generated by Glorp is displayed in the Transcript (with whitespace inserted for readability). What we see is the infamous "N+1 selects problem" in action - the first SELECT fetches the GROUPS rows, then, for each group_id, there is a corresponding SELECT to fetch the TEAMS rows.

SELECT t1.id, t1.created_at, t1.updated_at, t1.event_id, t1.title, t1.pos
 FROM GROUPS t1  an OrderedCollection()

SELECT t1.id, t1.created_at, t1.updated_at, t1.key, t1.title
 FROM TEAMS t1, GROUPS_TEAMS t2
 WHERE ((t2.team_id = t1.id) AND (t2.group_id = ?))  an OrderedCollection(1)

SELECT t1.id, t1.created_at, t1.updated_at, t1.key, t1.title
 FROM TEAMS t1, GROUPS_TEAMS t2
 WHERE ((t2.team_id = t1.id) AND (t2.group_id = ?))  an OrderedCollection(2)

SELECT t1.id, t1.created_at, t1.updated_at, t1.key, t1.title
 FROM TEAMS t1, GROUPS_TEAMS t2
 WHERE ((t2.team_id = t1.id) AND (t2.group_id = ?))  an OrderedCollection(3)

SELECT t1.id, t1.created_at, t1.updated_at, t1.key, t1.title
 FROM TEAMS t1, GROUPS_TEAMS t2
 WHERE ((t2.team_id = t1.id) AND (t2.group_id = ?))  an OrderedCollection(4)

SELECT t1.id, t1.created_at, t1.updated_at, t1.key, t1.title
 FROM TEAMS t1, GROUPS_TEAMS t2
 WHERE ((t2.team_id = t1.id) AND (t2.group_id = ?))  an OrderedCollection(5)

SELECT t1.id, t1.created_at, t1.updated_at, t1.key, t1.title
 FROM TEAMS t1, GROUPS_TEAMS t2
 WHERE ((t2.team_id = t1.id) AND (t2.group_id = ?))  an OrderedCollection(6)

SELECT t1.id, t1.created_at, t1.updated_at, t1.key, t1.title
 FROM TEAMS t1, GROUPS_TEAMS t2
 WHERE ((t2.team_id = t1.id) AND (t2.group_id = ?))  an OrderedCollection(7)

SELECT t1.id, t1.created_at, t1.updated_at, t1.key, t1.title
 FROM TEAMS t1, GROUPS_TEAMS t2
 WHERE ((t2.team_id = t1.id) AND (t2.group_id = ?))  an OrderedCollection(8)

Fortunately Glorp is cleverer than this, and provides a way to avoid the N+1 problem, by using the message #alsoFetch:.

| vh |
Transcript clear.
OFDatabase dbFileName: 'wc2018.db'
  evaluate: [ :db |
    | query |
    db session accessor logging: true.
    query := Query read: OFGroup.
    query alsoFetch: [ :ea | ea teams ]. " <== See me. "
    vh := String streamContents: [ :str | 
      (db session execute: query) do: [ :ea | 
        str nextPutAll: ea title; nextPut: Character cr.
        ea teams do: [ :team | 
          str nextPutAll: '- ', team title; nextPut: Character cr ]]]].
vh

Same output as before, but this time the SQL (pretty-printed by hand for readability) is much shorter and properly takes advantage of the SQL language.

SELECT t1.id, t1.created_at, t1.updated_at, t1.event_id, t1.title, t1.pos, 
       t2.id, t2.created_at, t2.updated_at, t2.key, t2.title
FROM GROUPS t1 
INNER JOIN GROUPS_TEAMS t3 ON (t1.id = t3.group_id) 
INNER JOIN TEAMS t2 ON (t3.team_id = t2.id) 
ORDER BY t1.id  an OrderedCollection()

Tags: Glorp, SQLite