![]() If you are curious, by typing rails generate (or the rails g shortcut), you will see all available generators. rails new sampleĬd sample Code language: Shell Session ( shell ) Creating a model Let’s start by creating a new application. Ruby on Rails has excellent tools for seeding a database, and thanks to the work of the community, many gems make this task easier.Īpart from seeding a database, we have helpful tools to check the database and better organize the data seeds.ĭo you want to see this article in video format? Creating a sample application See also my answer to Still Confused About Identifying vs.10 February 2021 Photo by Jeremy Bishop on Unsplash state is not part of the primary key of Person.Ī non-identifying relationship can be optional or mandatory, which means the foreign key column allows NULL or disallows NULL, respectively. But a row in Person is not identified by its state attribute. Person is a child table with respect to States. A good example of this is a lookup table, such as a foreign key on Person.state referencing the primary key of States.state. This is a strong clue that this is an identifying relationship (even if we don't literally include person_id in the primary key of PhoneNumbers).Ī non-identifying relationship is when the primary key attributes of the parent must not become primary key attributes of the child. We may think of the phone number(s) as belonging to a person, even though they are modeled as attributes of a separate table. Since we want to support multiple phone numbers, we make a second table PhoneNumbers, whose primary key includes the person_id referencing the Person table. If they had just one phone number, we could simply store it in a column of Person. But the logical relationship is that the child cannot exist without the parent.Įxample: A Person has one or more phone numbers. Formally, the "right" way to do this is to make the foreign key part of the child's primary key. This may be confusing because it's common practice these days to create a pseudokey for a child table, but not make the foreign key to the parent part of the child's primary key. select * from a FULL OUTER JOIN b on a.a = b.b Īn identifying relationship is when the existence of a row in a child table depends on a row in a parent table. If something in A doesn't have a corresponding datum in B, then the B portion is null, and vice versa. Select a.*, b.* from a,b where a.a(+) = b.b Ī full outer join will give you the union of A and B, i.e. select * from a RIGHT OUTER JOIN b on a.a = b.b Select a.*, b.* from a,b where a.a = b.b(+) Ī right outer join will give all rows in B, plus any common rows in A. select * from a LEFT OUTER JOIN b on a.a = b.b Select a.*, b.* from a,b where a.a = b.b Ī left outer join will give all rows in A, plus any common rows in B. select * from a INNER JOIN b on a.a = b.b Note that (1,2) are unique to A, (3,4) are common, and (5,6) are unique to B.Īn inner join using either of the equivalent queries gives the intersection of the two tables, i.e. Suppose you have two tables, with a single column each, and data as follows: A B the inner part of a Venn diagram intersection.Īn outer join of A and B gives the results of A union B, i.e. I just checked the source and the dependencies are like this now:ĭb:create creates the database for the current envĭb:create:all creates the databases for all envsĭb:drop drops the database for the current envĭb:drop:all drops the databases for all envsĭb:migrate runs migrations for the current env that have not run yetĭb:migrate:up runs one specific migrationĭb:migrate:down rolls back one specific migrationĭb:migrate:status shows current migration statusĭb:rollback rolls back the last migrationĭb:forward advances the current schema version to the next oneĭb:schema:load loads the schema into the current env's databaseĭb:schema:dump dumps the current env's schema (and seems to create the db as well)ĭb:migrate:redo runs (db:migrate:down db:migrate:up) or (db:rollback db:migrate) depending on the specified migrationĭb:migrate:reset runs db:drop db:create db:migrateįor further information please have a look at (for Rails 3.2.x) and (for Rails 4.0.x)Īssuming you're joining on columns with no duplicates, which is a very common case:Īn inner join of A and B gives the result of A intersect B, i.e. db:schema:load is used when you setup a new instance of your app. Typically, you would use db:migrate after having made changes to the schema via new migration files (this makes sense only if there is already data in the database). This will delete existing data.ĭb:setup does db:create, db:schema:load, db:seedĭb:migrate:reset does db:drop, db:create, db:migrate Db:migrate runs (single) migrations that have not run yet.ĭb:schema:load creates tables and columns within the existing database following schema.rb.
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