Introduction to Entities

If you've heard something about ODK Entities and want to better understand whether they're useful for your longitudinal data collection, you're in the right place! We've organized this page as a series of questions that are independent from each other so you can pick and choose the sections that interest you.

If you're someone who learns best by doing, you may prefer to jump straight into the tutorial on building a community reporting tool with Entities and to come back here if you have any questions. If you have a question that we haven't answered, you can post on the forum.


This document assumes you are using ODK Cloud or an up-to-date ODK Central server. Entities are not yet available in other ODK-compatible systems.


What are Entities?

In the ODK context, an "Entity" can be thought of as a "thing". If your project involves things that need to be shared between forms and may change over time, you can represent them as Entities. You can use Entities to represent real things like trees, people, or cities. You can also represent more abstract things like tree visits, malaria cases, or city ratings as Entities.

Entities are organized in Entity Lists that group together Entities of the same type. You can think of Entity Lists as spreadsheets or databases that are shared across forms. Forms can create, read, and update Entities. You can also think of Entities as the nouns (trees) and the forms as the verbs (Register a tree).

ODK has historically been form-based: every workflow starts by opening a blank form and filling it out. We are now working towards providing an Entity-based option in which workflows start by selecting an Entity. Currently, you can have each of your form start with a question that lets users select an Entity from a list. In the future, you'll have the option to first select an Entity and then see what forms, if any, apply to that Entity.


Can you show me how it works?

Sure! Here's a video that shows a survey with baseline and follow-up that uses Entities.

Should I use Entities?

Entities are an optional add-on to ODK. There are many users of ODK who don't need to use Entities because their data collection is done at a single moment in time and doesn't use any previously collected data.

Any workflow that involves multiple steps over time has the potential to benefit from Entities. If you find yourself wishing you could flow data from one form to another or send data back to devices, you likely want to use Entities. Common examples include site inspections, studies that involve linking baseline and follow-up visits, and case management.

Entities can also be helpful for sharing data that rarely changes between multiple forms. For example, if all your forms need to use a country's districts, you can represent those once in an Entity List and share that list between the forms. This means if the districts do change, you can make the necessary updates in one place and know that all related forms will get the update.

Entities are very powerful and it can be tempting to look for ways to use them in every project. This power comes at a cost of greater complexity and more potential for error, though. When you use Entities, you have to consider the possibility that some users may be offline for some time, possibly resulting in conflicts. You also have to consider that Entities themselves will change over time and affect form design logic in ways you may not expect.

Why can't I just flow data from one form to another form?

We have added the Entity concept instead of letting data flow directly between forms because it adds more flexibility. In particular, it's common to have a workflow centered around a thing with a status that determines what needs to be done with that thing. Having an Entity representation with one or more properties that represent its status means it's significantly easier to have multiple forms that can update that status and to show a list of Entities with the latest status information.

In many contexts, the information that needs to be shared between forms is minimal and sometimes as little as an ID and label are enough. Sometimes the subjects of a workflow are known ahead of time, either from a prior ODK form or some other system. Entities makes both of these cases straightforward to represent.

If your workflow requires accessing all captured data about an Entity, directly flowing data between forms would likely have worked well. You can achieve something similar with Entities by creating an Entity List that represents encounters with the Entity.

For example, let's say that you have trees that you want to evaluate over time. You could have a trees Entity List that includes fixed properties of the trees: their location, their species, etc. Then you could have a second Entity List called tree_measurements that includes a property that represents a link back to a tree Entity as well as any measurements made during a new encounter.


I filled out a registration form and don't immediately see my Entity in follow-up forms, why?

Currently, in order for a submission to create or update an Entity, that submission has to be processed by your server. That means that if you create a new Entity or update an existing one by filling out a form, you won't see that change reflected in follow-up forms until you download the latest update to your Entity List from your server.

If you usually have Internet connectivity, this is unlikely to be very important. Similarly, if your registration and follow-up periods happen at very different times, this limitation is not a problem. But for workflows in which follow-up needs to happen immediately after registration or multiple follow-ups are needed while offline, this limitation is significant.

Offline Entity support is expected in late 2024, read more on the forum.

I need to assign specific Entities to specific data collectors, how can I represent this?

Currently, an entire Entity List is always sent to every device and there is no way to subset the list. This is something that we intend to eventually enable.

For now, you can limit the Entities that are available from a select_one_from_file using a choice_filter. This won't limit the amount of data sent to each device but it can significantly reduce the amount of options shown to each user and can help speed up lookup expressions.

Can I have millions of Entities?

There are two current limitations that make millions of Entities impractical: data transfer and form performance.

Currently, all Entities that have not been deleted are sent to every device on every update. Depending on your data connection, this may be a limiting factor for your project. We will eventually add support for archiving Entities to address this limitation.

Entities are currently represented in memory for access by forms. Modern devices can easily process multiple tens of thousands of entities in this way, but your form may become slow or crash if you have more than 50,000 Entities.

We are actively working on addressing these performance limitations and expect significant improvements by late 2024. In the mean time, one possible workaround is to use pulldata and search() instead of instance and select_one_from_file. These methods are less flexible but they will perform better.

My form captures data on multiple different things, can I create multiple Entities with a single submission?

Not yet, but this is something we will eventually support.

If you find yourself wanting to create or update multiple Entities of the same type in a repeat, your best option currently is to use multiple submissions of the same form instead of a repeat. You can capture base information in one form and then use a separate form to create each Entity that you currently represent by repeat instances.

If there is a parent-child relationship between the different Entities, you can save the parent's ID to each child. If you are working in an environment with Internet connectivity, you can refresh the forms to see your created parent Entities in your child Entity creation forms. If you are working in a disconnected environment, you can have data collectors copy the ID from the parent form to the child forms.

Similarly, if you'd like to establish relationships between multiple Entities of different types, you can have a registration form for each type and include a field to represent a link to another Entity.


What's the difference between Entities and CSV form attachments?

From a form design perspective, they are identical. That means you can attach them to forms, look values up in them or build selects on them in the exact same way.

From a server perspective, a CSV form attachment can only be associated with a single form, unlike Entities which can be shared between forms. CSV form attachments are stored as files and if you need to update one row in a CSV attachment, you need to replace the whole file. In contrast, Entities can be updated individually.

You can – and many users do – accomplish the same thing as Entities with CSV form attachments and your own automation using the Central API. The biggest advantage of Entities over that approach is that you don't need to run your own automation.

I use CSV form attachments for longitudinal data collection, should I use Entities instead?

If CSV form attachments are working well for you, you don't need to change anything. In particular, if your workflow involves distinct phases such as annual data collection events, it may be better to analyze and clean baseline data before feeding it into the next phase rather than automatically flowing data with Entities.

If there's a need to periodically update your CSV form attachment, you may want to consider using Entities to save time and reduce the opportunity for mistakes that can come from a manual process such as forgetting to update or attaching the wrong file.

What's the difference between Entities and choice lists?

From a form design perspective, they are nearly identical. The only significant difference is that because Entity Lists are defined outside of a form, you need to explicitly attach them to your forms using select_*_from_file or csv-external. Another difference is that there currently isn't support for media or translations in Entity Lists. Other than that, the way that you look up values in choice lists and Entity Lists using instance() is identical.

Can ODK now replace more specialized software?

ODK is a flexible data collection platform. Its strength is that it lets you quickly build forms that meet your exact needs. With Entities, you can now think of ODK as an application-building platform. With data defined by your Entity Lists and behavior defined by your forms, you have the freedom to represent only the things that matter to you and to define exactly what actions can be taken on them.

The domain that you work in likely has systems for managing workflows similar to the ones you need to support. This could be a system designed to support a community health worker program, to monitor tree health over time, to track samples in a lab, etc. Those systems typically have some built-in concepts around the data that needs to be collected, the people that might be involved, the status changes that a workflow subject can go through, and so on.

If you have specialized software that supports your domain's workflows, we recommend giving that software a try. If instead you prefer the flexibility to define your forms and Entity Lists to exactly match your workflow needs, ODK will be better a fit.

Here are some questions to consider when deciding between using ODK and specialized software:

  • Which platform provides the data collection features I need? If you need powerful features like custom logic, offline basemaps, and barcode scanning, ODK has that and more.

  • How easy is it for me to support the basic concepts of my workflow? For example, implementing patient transfers between health workers is possible in ODK, but it requires a lot of work.

  • Are my workflows mostly data collection or mostly something else? If your workflows are primarily driven by data collection, you'll likely be better served with ODK.

In general, workflows that are focused or short-lived can very easily be represented in ODK and may not benefit from a system made specifically for that purpose. More wide-ranging or long-lived workflows are more likely to benefit from a more structured and specialized system.

That said, in many contexts, workflow needs are so specific that a flexible platform like ODK offers great benefits. Once you have defined your workflow in ODK, the forms you have built can become the standard, specialized way to support others in your domain.


How do I access Entities from my forms?

First, attach the Entity List you want to access Entities from in your form definition either using select_one_from_file or csv-external.

If you want the user to be able to select an Entity from a list, you can use a select_one_from_file question with the name of your Entity List followed by .csv. For example, if your Entity List is named trees, you would have a select_one_from_file trees.csv question.

Everything you know about selects and selects from files apply to attached Entity Lists. For example, you can use an Entity property in a choice_filter expression to filter down an Entity List.

If you want to look up Entities using a user-provided value such as a unique ID scanned from a barcode or entered manually, you can attach your Entity List with csv-external.

Once a specific Entity is identified, you can look up its properties using a lookup expression. All of this works exactly the same way as it does with CSV form attachments.

How do I use forms to create or update Entities?

Add an entities sheet to your form and use it to define the Entity List that the form's submission will populate and an expression for each Entity's label. Next, specify which form fields should be saved to Entity properties by putting the desired property name in the save_to column for each form field.

What form fields should I save to my Entities as properties?

This will vary a lot project by project. In many cases, a descriptive label clearly identifying individual Entities is enough to meet goals like making sure that no duplicate Entities are created and connecting submissions about the same Entity in analysis.

For more complex workflows, it can be helpful to include a property that represents a status which determines what forms can operate on any given Entity. In some contexts, it may be important to include multiple identifying properties to make sure that the correct Entity is selected. Sometimes it's important to show data collectors a summary of information that was previously captured and so it must all be saved on the Entity.

We recommend thinking carefully about the minimum amount of data that you need to drive your workflow. The less data you save and access, the simpler your form design will be and the less data will need to be transmitted to data collectors. However, there is no enforced limit on number of properties.

Currently, once a property is added to an Entity List, it can't be removed. You can stop writing data to that column and ignore it in follow-up forms but you can't delete it. We will eventually add support for archiving Entities.

What are Entity conflicts and what can I do to avoid them?

A conflict happens when two form submissions that are received by the server have updates to the same Entity with the same version. If the two submissions specify different, overlapping updates to one or more properties, the server will provide an interface for resolving it. All conflicts have to be explicitly dismissed.

One of our goals with Entities is to let field staff make as much progress as possible without interruption so the server applies conflicting updates with the latest one taking precedence. The conflict is shown on the server and office staff can look at the submitted data and work with field staff to resolve the issue.

When possible, we recommend using Entity properties and a choice_filter to limit the number of Entities that a specific field worker sees. This will greatly reduce the chance of conflicts.

Should I analyze Entity data, form submission data or both?

Which is most appropriate will depend on the goals of your project.

Entities can be very useful for tracking work completion. Computing counts of Entities or of Entities of a particular status can be a simple way to understand project status. This can be useful independent of how final data analysis is conducted.

When the goal of a project is to deliver a service or to understand the final state of some Entities, it may be most practical to analyze the data in the Entities themselves.

Many projects involve capturing in-depth survey data at multiple points in time. In those cases, it's not important and can even be undesirable for historical data to be sent back to devices as Entities. In those cases, Entities can be used to drive the workflow and analysis can be conducted on form submission data, using Entity IDs to link submissions to each other.

Can I import data from another system as Entities?

Yes, you can import Entities to an existing Entity List by uploading a CSV or using the API.

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