Most databases allow defining UNIQUE key (unique field) that is not a PRIMARY KEY, but DynamoDB does not seem to support unique key definitions.
For example, a model SampleModel defines an id field as a PRIMARY KEY (id = UnicodeAttribute(hash_key=True)). What if another field (let's say name) must also be defined as unique? Given that DynamoDB does not offer unique field specification, and only one PK (hash_key=True) is allowed - how can name be defined as UNIQUE?
As you've already seen, there's no direct support for that in DynamoDB.
If this name attribute is immutable after you write it, you could do the following:
You can create a second DynamoDB table that only has a Partition Key (Partition Key = Primary Key) and stores each name there. When you add an item to the first table, you use a transaction and have a separate insert into the second table that has a condition of the key not already existing. If this transaction fails, you were trying to put an item whose name already existed in the table. If the transaction goes through, you've inserted a new unique name.
This will incur extra cost for the second table and transactions and only works in this narrow use case.
DynamoDB is not built for this pattern, enforce uniqueness in another way.
You create a PK out of whatever value has to be unique (in the same table or second table) and use transactions to enforce it on insert.
https://aws.amazon.com/blogs/database/simulating-amazon-dynamodb-unique-constraints-using-transactions/
I have created tables in MySQL Workbench as shown below :
ORDRE table:
CREATE TABLE Ordre (
OrdreID INT NOT NULL,
OrdreDato DATE DEFAULT NULL,
KundeID INT DEFAULT NULL,
CONSTRAINT Ordre_pk PRIMARY KEY (OrdreID),
CONSTRAINT Ordre_fk FOREIGN KEY (KundeID) REFERENCES Kunde (KundeID)
)
ENGINE = InnoDB;
PRODUKT table:
CREATE TABLE Produkt (
ProduktID INT NOT NULL,
ProduktBeskrivelse VARCHAR(100) DEFAULT NULL,
ProduktFarge VARCHAR(20) DEFAULT NULL,
Enhetpris INT DEFAULT NULL,
CONSTRAINT Produkt_pk PRIMARY KEY (ProduktID)
)
ENGINE = InnoDB;
and ORDRELINJE table:
CREATE TABLE Ordrelinje (
Ordre INT NOT NULL,
Produkt INT NOT NULL,
AntallBestilt INT DEFAULT NULL,
CONSTRAINT Ordrelinje_pk PRIMARY KEY (Ordre, Produkt),
CONSTRAINT Ordrelinje_fk FOREIGN KEY (Ordre) REFERENCES Ordre (OrdreID),
CONSTRAINT Ordrelinje_fk1 FOREIGN KEY (Produkt) REFERENCES Produkt (ProduktID)
)
ENGINE = InnoDB;
so when I try to insert values into ORDRELINJE table i get:
Error Code: 1452. Cannot add or update a child row: a foreign key constraint fails (srdjank.Ordrelinje, CONSTRAINT Ordrelinje_fk FOREIGN KEY (Ordre) REFERENCES Ordre (OrdreID))
I've seen the other posts on this topic, but no luck.
Am I overseeing something or any idea what to do?
Taken from Using FOREIGN KEY Constraints
Foreign key relationships involve a parent table that holds the
central data values, and a child table with identical values pointing
back to its parent. The FOREIGN KEY clause is specified in the child
table.
It will reject any INSERT or UPDATE operation that attempts to create
a foreign key value in a child table if there is no a matching
candidate key value in the parent table.
So your error Error Code: 1452. Cannot add or update a child row: a foreign key constraint fails essentially means that, you are trying to add a row to your Ordrelinje table for which no matching row (OrderID) is present in Ordre table.
You must first insert the row to your Ordre table.
The Problem is with FOREIGN KEY Constraint. By Default (SET FOREIGN_KEY_CHECKS = 1). FOREIGN_KEY_CHECKS option specifies whether or not to check foreign key constraints for InnoDB tables. MySQL - SET FOREIGN_KEY_CHECKS
We can set foreign key check as disable before running Query. Disable Foreign key.
Execute one of these lines before running your query, then you can run your query successfully. :)
1) For Session (recommended)
SET FOREIGN_KEY_CHECKS=0;
2) Globally
SET GLOBAL FOREIGN_KEY_CHECKS=0;
This error generally occurs because we have some values in the referencing field of the child table, which do not exist in the referenced/candidate field of the parent table.
Sometimes, we may get this error when we are applying Foreign Key constraints to existing table(s), having data in them already. Some of the other answers are suggesting to delete the data completely from child table, and then apply the constraint. However, this is not an option when we already have working/production data in the child table. In most scenarios, we will need to update the data in the child table (instead of deleting them).
Now, we can utilize Left Join to find all those rows in the child table, which does not have matching values in the parent table. Following query would be helpful to fetch those non-matching rows:
SELECT child_table.*
FROM child_table
LEFT JOIN parent_table
ON parent_table.referenced_column = child_table.referencing_column
WHERE parent_table.referenced_column IS NULL
Now, you can generally do one (or more) of the following steps to fix the data.
Based on your "business logic", you will need to update/match these unmatching value(s), with the existing values in the parent table. You may sometimes need to set them null as well.
Delete these rows having unmatching values.
Add new rows in your parent table, corresponding to the unmatching values in the child table.
Once the data is fixed, we can apply the Foreign key constraint using ALTER TABLE syntax.
You are getting this constraint check because Ordre table does not have reference OrdreID provided in insert command.
To insert value in Ordrelinje, you first have to enter value in Ordre table and use same OrdreID in Orderlinje table.
Or you can remove not null constraint and insert a NULL value in it.
You must delete data in the child table which does not have any corresponding foreign key value to the parent table primary key .Or delete all data from the child table then insert new data having the same foreign key value as the primary key in the parent table . That should work .
This helped me out after reading #Mr-Faizan's and other answers.
Untick the 'Enable foreign key checks'
in phpMyAdmin and hit the query.
I don't know about WorkBench but the other answers might help you out.
I had the same problem. I was creating relationships on existing tables but had different column values, which were supposed/assumed to be related. For example, I had a table USERS that had a column USERID with rows 1,2,3,4,5. Then I had another child table ORDERS with a column USERID with rows 1,2,3,4,5,6,7. Then I run MySQl command ALTER TABLE ORDERS ADD CONSTRAINT ORDER_TO_USER_CONS FOREIGN KEY (ORDERUSERID) REFERENCES USERS(USERID) ON DELETE SET NULL ON UPDATE CASCADE;
It was rejected with the message:
Error Code: 1452. Cannot add or update a child row: a foreign key constraint fails (DBNAME1.#sql-4c73_c0, CONSTRAINT ORDER_TO_USER_CONS FOREIGN KEY (ORDERUSERID) REFERENCES USERS (USERID) ON DELETE SET NULL ON UPDATE CASCADE)
I exported data from the ORDERS table, then deleted all data from it, re-run the command again, it worked this time, then re-inserted the data with the corresponding USERIDs from the USERS table.
in the foreign key table has a value that is not owned in the primary key table that will be related, so you must delete all data first / adjust the value of your foreign key table according to the value that is in your primary key
I found that changing the foreign key back from not null to null BEFORE I tried to do what I knew was the correct code, got it working. Helped that I was using Mysql workbench. I had to also set SET FOREIGN_KEY_CHECKS=0; and then back to =1; after finished.
This can be fixed by inserting the respective records in the Parent table first and then we can insert records in the Child table's respective column. Also check the data type and size of the column. It should be same as the parent table column,even the engine and collation should also be the same.
TRY THIS! This is how I solved mine. Correct me if am wrong.
In my case the tables were perfectly consistent.
Anyway I was getting this error because I created (by accident) more than one FK constraint on the same field.
I run the following query to show all the keys:
SELECT *
FROM information_schema.table_constraints
WHERE constraint_schema = 'my_db_name'
and I deleted the wrong ones with the following query:
ALTER TABLE my_table
DROP FOREIGN KEY wrong_fk_constraint;
You can check it also running this query:
SHOW CREATE TABLE my_table;
While inserting the foreign key attribute values, first verify the attributes type, as well as primary key attribute value in the parent relation, if the values in parent relation matches, then you can easily insert/update child attribute values.
I was getting this issue even though my parent table had all the values I was referencing in my child table. The issue seemed to be that I could not add multiple child references to a single foreign key. In other words if I had five rows of data referenced the same foreign key, MySQL was only allowing me to upload the first row and giving me the error 1452.
What worked for me was typing the code "SET GLOBAL FOREIGN_KEY_CHECKS=0". After that I closed out of MySQL and then restarted it and I was able to upload all of my data with no errors. I then typed "SET GLOBAL FOREIGN_KEY_CHECKS=1" to set the system back to normal although I'm not entirely sure what FOREIGN_KEY_CHECKS does. Hope this helps!
First allow NULL on the parent table and set the default values to NULL. Next create the foreign key relationship. Afterwards, you can update the values to match accordingly
For PhpMyAdmin , Go to the structure of table where you created foreign key and then click on Relation view , in that choose No Action under on Delete and on Update section.
Note : this work for me.
enter image description here
Your ORDRELINJE table is linked with ORDER table using a foreign key constraint constraint Ordrelinje_fk foreign key(Ordre) references Ordre(OrdreID) according to which Ordre int NOT NULL, column of table ORDRELINJE must match any Ordre int NOT NULL, column of ORDER table.
Now what is happening here is, when you are inserting new row into ORDRELINJE table, according to fk constraint Ordrelinje_fk it is checking ORDER table if the OrdreID is present or not and as it is not matching with any OrderId, Compiler is complaining for foreign key violation. This is the reason you are getting this error.
Foreign key is the primary key of the other table which you use in any table to link between both. This key is bound by the foreign key constraint which you specify while creating the table. Any operation on the data must not violate this constraint. Violation of this constraint may result in errors like this.
Hope I made it clear.
you should insert at least one raw in each tables (the ones you want the foreign keys pointing at) then you can insert or update the values of the foreign keys
you should add data from REFERENCES KEY in PRIMARY TABLE to FOREIGN KEY in CHILD TABLE
it means do not add random data to foreign key ، just use data from primary key that is accessable
description of data in foreign key
The problem occurs because you set the foreign key in child table after you insert some data in the child table.
Try removing all the data from child table, then set the foreign key and afterwards add/insert the data in table, it will work.
check the no. of record in parent table that matches with child table and also primary key must match with foreign key reference.
This works for me.
I am having the same issue here is my scenario
i put empty('') where value is NULL
now this '' value does not match with the parent table's id
here is things need to check , all value with presented in parent table
otherwise remove data from parent table then try
ill squeeze this in here:
my case was trying to create a like for a post which dint exist;
while committing to database the error was raised.
solution was to first create the post then like it.
from my understanding if the post_id was to be saved in the likes table it had to first check with posts table to ascertain existence.
i found it better to have it this way since its more logical to me that way..
When you're using foreign key, your order of columns should be same for insertion.
For example, if you're adding (userid, password) in table1 from table2 then from table2 order should be same (userid, password) and not like (password,userid) where userid is foreign key in table2 of table1.
The answer of your question is that you must set the same value in Primary and secondary key.
Thanks
Actually, i solved just like this "insert into databasename.tablename" it worked. And after when i try to add data like "insert into databasename" it worked to.
Just something else to look for. If you had to delete records from one of your tables and are expecting the values to start at 1, you could get this error. The solution was to run a SHOW * FROM tablename on all the Parent tables. When I did I noticed in one of the tables where I had had a problem earlier and had to delete some records that the primary key values were not what I was expecting them to be from a previous SELECT *.
Probably better answered above, but when working in mysql workbench you don't need to commit the transaction immediatly, you can commit the parent and child element at the same time. So setup the parent with sql or in the gui and add the child in the gui / sql and commit concurrently.
If working in code and getting this issue you can create a factory to create the parent and then create the child / join.
Theoretically you would need an Order to have an OrderId thus create an Order. The create an OrderId and that OrderId may have a number of associated products which you can then add to the OrderId or do with as you wish.
I am trying to select a subset of columns from a table with sqlalchemy's load_only function. Unfortunately it doesn't seem to return only the columns specified in the functional call - specifically, it also seems to fetch the primary key (in my case, an auto_increment id field).
A simple example, if I use this statement to build a query,:
query = session.query(table).options(load_only('col_1', 'col_2'))
Then the query.statement looks like this:
SELECT "table".id, "table"."col_1", "table"."col_2"
FROM "table"
Which is not what I would have expected - given I've specified the "only" columns to use...Where did the id come from - and is there a way to remove it?
Deferring the primary key would not make sense, if querying complete ORM entities, because an entity must have an identity so that a unique row can be identified in the database table. So the query includes the primary key though you have your load_only(). If you want the data only, you should query for that specifically:
session.query(table.col1, table.col2).all()
The results are keyed tuples that you can treat like you would the entities in many cases.
There actually was an issue where having load_only() did remove the primary key from the select list, and it was fixed in 0.9.5:
[orm] [bug] Modified the behavior of orm.load_only() such that primary key columns are always added to the list of columns to be “undeferred”; otherwise, the ORM can’t load the row’s identity. Apparently, one can defer the mapped primary keys and the ORM will fail, that hasn’t been changed. But as load_only is essentially saying “defer all but X”, it’s more critical that PK cols not be part of this deferral.
I have an unusual challenge. I'm modifying a table to be able to join with two other legacy groups of PostgreSQL tables.
One group pretty much requires that each record in the table have a unique integer. So, the following field definition would work:
numeric_id = sql.Column(sql.Integer, primary_key=True)
The other group of tables all use UUID fields for the expected JOIN requests. So the following field definition would work:
uu_account_id = sql.Column(UUID(as_uuid=True), primary_key=True)
But, clearly, I can't have two primary keys. So one of them needs to not be a primary key. It would be nice to simply have both still be automatically assigned when a new record is made.
Any suggestions?
I'm sure I can do a quick hack, but I'm curious if there is a nice clean answer.
(And no: changing the other tables is NOT an option. Way too much legacy code.)
Make the uuid column the primary key, like usual.
Define the other column as having serial type and unique. In SQL I'd write
create table mytable (
mytable_id uuid primary key default uuid_generate_v4(),
mytable_legacy_id serial unique not null,
... other cols ...
);
so you just need to do the SQLAlchemy equivalent, whatever that is, of a not null, unique field.
Note that "serial" is just shorthand for
create sequence tablename_colname_seq;
create table tablename (
colname integer default nextval('tablename_colname_seq'),
... cols ...
);
alter sequence tablename_colname_seq owned by tablename.colname;
so if you can't make sqlalchemy recognise that you can have a serial field that isn't a primary key, you can do it this way instead.
Between the SQLAlchemy, alembic (which I also use), and PostgreSQL, this turned out to be tricky.
If creating a brand new table from scratch, the following works for my numeric_id column:
numeric_id = sql.Column(sql.Integer, sql.Sequence('mytable_numeric_id_seq'), unique=True, nullable=False)
(It is possible that the unique=True and nullable=False are overkill.)
However, if modifying an existing table, the sequence itself fails to get created. Or, at least, I couldn't get it to work.
The sequence can be created by hand, of course. Or, if using 'alembic' to make distributed migrations easier, add:
from sqlalchemy.schema import Sequence, CreateSequence
def upgrade():
op.execute(CreateSequence(Sequence("mytable_numeric_id_seq")))
To the version script created by alembic.
Special thanks to Craig for his help.
(Note: most of the SQLAlchemy examples on the net use db. as the module alias rather than sql.. Same thing really. I used sql. simply because I'm using db. already for MongoDB.)
I am trying to use a Django model to for a record but then return a concatenated field of two different tables joined by a foreign key.
I can do it in SQL like this:
SELECT
location.location_geoname_id as id,
CONCAT_WS(', ', location.location_name, region.region_name, country.country_name) AS 'text'
FROM
geonames_location as location
JOIN
geonames_region as region
ON
location.region_geoname_id = region.region_geoname_id
JOIN
geonames_country as country
ON
region.country_geoname_id = country.country_geoname_id
WHERE
location.location_name like 'location'
ORDER BY
location.location_name, region.region_name, country.country_name
LIMIT 10;
Is there a cleaner way to do this using Django models? Or do I need to just use SQL for this one?
Thank you
Do you really need the SQL to return the concatenated field? Why not query the models in the usual way (with select_related()) and then concatenate in Python? Or if you're worried about querying more columns than you need, use values_list:
locations = Location.objects.values_list(
'location_name', 'region__region_name', 'country__country_name')
location_texts = [','.join(l) for l in locations]
You can also write raw query for this in your code like that and later on you can concatenate.
Example:
org = Organization.objects.raw('SELECT organization_id, name FROM organization where is_active=1 ORDER BY name')
Keep one thing in a raw query you have to always fetch primary key of table, it's mandatory. Here organization_id is a primary key of contact_organization table.
And it's depend on you which one is useful and simple(raw query or model query).