Now, let's demonstrate how the cascade delete works. In this example, we've created a foreign key (with cascade delete) called fk_departments that references the departments table based on the department_id field. INSERT INTO employees SELECT * FROM _employees_old Now, let's add a foreign key with cascade delete to the employees table: PRAGMA foreign_keys=off ĪLTER TABLE employees RENAME TO _employees_old INSERT INTO employees VALUES (10001, 'Anderson', 'Dave', 999) INSERT INTO employees VALUES (10000, 'Smith', 'John', 30) INSERT INTO departments VALUES (999, 'Sales') By default, Room uses the class name as the database table name. You can make sure Room has access to a field either by making it public or by providing getter and setter methods for it. INSERT INTO departments VALUES (30, 'HR') PrimaryKey val id: Int, val firstName: String, val lastName: String ) Note: To persist a field, Room must have access to it. Next, let's add some data to these tables: INSERT INTO table1 SELECT * FROM _table1_old įirst, let's start by creating our 2 tables ( departments and employees): The syntax to add a foreign key with cascade delete to an existing table in SQLite is: PRAGMA foreign_keys=off ĪLTER TABLE table1 RENAME TO _table1_old Instead you will need to rename the table, create a new table with the foreign key, and then copy the data into the new table. You can not use the ALTER TABLE statement to add a foreign key with cascade delete in SQLite. How to Add a Foreign Key with Cascade Delete to an Existing Table Then we've created a foreign key called fk_departments on the employees table that references the departments table based on the department_id field.īecause of the cascade delete, when a record in the departments table is deleted, all records in the employees table will also be deleted that have the same department_id value. In this example, we've created a primary key on the departments table that consists of only one field - the department_id field. ( employee_id INTEGER PRIMARY KEY AUTOINCREMENT, ( department_id INTEGER PRIMARY KEY AUTOINCREMENT, Let's look at an example of how to create a foreign key with cascade delete using the CREATE TABLE statement in SQLite. REFERENCES parent_table (column1, column2. The syntax for creating a foreign key with cascade delete using a CREATE TABLE statement in SQLite is: CREATE TABLE table_nameįOREIGN KEY (column1, column2. It's too bad explain can't be used as a sub-query though.How to Create a Foreign Key with Cascade Delete using a CREATE TABLE statement Syntax Sqlite> explain select x from separatePK not indexed ġ OpenRead 0 4 0 1 00 root=4 iDb=0 separatePK Sqlite> explain select x from rowidPK not indexed ġ OpenRead 0 3 0 0 00 root=3 iDb=0 rowidPKĨ Transaction 0 0 10 0 01 usesStmtJournal=0 Sorry I haven't had the patience to create code examples. This runs slower and less efficiently than a magical INTEGER PRIMARY KEY / ROW ID alias, but at least it works.Īnyway, I hope that's helpful for developers working to improve linq2db. The simple work around for linq2db users, if they are free to edit their database schema, is to set the primary key to "autoincrement". So you'd just have to re-implement the heuristics of SQLite's decision logic found in their " Create Table" document. It short, it seems there's currently no direct way short of writing an extension method for SQLite in C. I've written up some details about how I've tried to figure out how to extract this piece of metadata directly from the database on another github project. The main difficulty I foresee in fixing this is that SQLite does not expose whether a table is using an INTEGER PRIMARY KEY, or whether it is just an INTEGER column which also happens to be a PRIMARY KEY. The user should be able to leave them as null when inserting new rows and find each is given a different ID, as provided by SQLite. Linq2db should really treat these special "INTEGER PRIMARY KEY" (RowIDs) more or less the same as an autoincrement column. This causes every inserted row to overwrite the last. Linq2db, unfortunately, believes this, and so doesn't let the user leave the column blank or null, unhelpfully setting the PK to 0 during insert. SQLite's metadata does not tell linq2db that these special primary keys might be nullable, nor does it flag them with "autoincrement". Linq2db's LoadSQLiteMetadata() does not pick up on when a column is a ROW ID alias. More details about SQLite's ROW ID alias columns are at the " Create Table" documentation. But it executes faster and can be searched faster. It differs from a "true" autoincrementing field in that IDs of deleted rows are re-used. These columns act something like an auto-increment column, so that when it's left empty or NULL during an insert, SQLite gives it a value automatically. These columns act differently to a regular INTEGER column, or to a BIGINT PRIMARY KEY column (despite BIGINT being otherwise synonymous with INTEGER). When a table has an "INTEGER PRIMARY KEY" then that column might become an alias for ROW ID. SQLite tables have a hidden "ROW ID" column.
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